Search

Various intermediates for the synthesis of erythronolide A, an aglycon of erythromycin A, are prepared from the corresponding seco-acids using 2-methyl-6-nitrobenzoic anhydride (MNBA) in the presence of 4-(dimethylamino)pyridine (DMAP) with or without triethylamine. The efficiency of the MNBA lactonization is assessed by studying this method and comparing the results with those of the other established macrocyclization protocols. It has been finally concluded that (i) the conformationally appropriate substrate for the monomeric cyclization gave the desired lactone in excellent yield under mild reaction conditions in the presence of MNBA and DMAP, (ii) the highly-strained substrate for the cyclization also afforded the monomeric lactone in relatively good yield at 100°C in toluene, and (iii) the seco-acid having stable linear conformation, which preferred dimerizing more than forming the monomeric lactone, provided the corresponding diolide in high yield with the constant ratio of the monomer to dimeric lactone (approximately 1/5). © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 305–320; 2009: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.200900017

We have developed effective methods for the total synthesis of 2-hydroxytetracosanolide and 2-hydroxy-24-oxooctacosanolide, the defensive salivary secretions of termites. The former natural product isolated from Armitermes neotenicus, a species of termite that inhabits Guyana, contains a 25-membered lactone backbone, and the latter, extracted from Pseudacanthoterme springer, an African termite, includes a 29-membered lactone moiety. The key cyclization to produce the 25- or 29-membered lactone core is performed by using 2-methyl-6-nitrobenzoic anhydride (MNBA) with a stoichiometric amount of 4-(dimethylamino)pyridine (DMAP) or a catalytic amount of 4-(dimethylamino)pyridine N-oxide (DMAPO).

In the proline-mediated Mannich and aldol reactions of propanal as a nucleophile, the aldimine prepared from benzaldehyde and p-anisidine is about 7 times more reactive than the corresponding alde- hyde, benzaldehyde, as an electrophile. This higher reactivity of aldimine over aldehyde is attributed to the carboxylic acid of proline protonating the basic ni- trogen atom of the aldimine more effectively than the oxygen atom of the aldehyde, an explanation which has been both experimentally and theoretically veri- fied.

Three types of sugars, 6-deoxy-L-tallose,4-C-methyl-D-ribose, and D-ribose were synthesized from both achiral starting materials, α,β-unsaturated aldehydes and silyl enol ethers of α-benzyloxy thioesters, by successive asymmetric aldol and vicinal dihydroxylation reactions.

In the presence of a catalytic amount of Lewis acid, various carboxylic esters or S-phenyl carbothioates are prepared in excellent yields by the respective reactions of equimolar amounts of silyl carboxylates and alkyl silyl ethers or phenyl silyl sulfides with 4-trifluoromethylbenzoic anhydride.

In the presence of a catalytic amount of titanium(IV) salt, various S-phenyl carbothioates are prepared in excellent yields by the reaction of nearly equimolar amounts of silyl derivatives of carboxylic acids and benzenethiols with p-trifluoromethylbenzoic anhydride.

Various carboxylic esters are prepared in excellent yields from nearly equimolar amounts of free carboxylic acids and alcohols at room temperature by combined use of 4-(trifluoromethyl)benzoic anhydride and a catalytic amount of active titanium(IV) salt together with chlorotrimethylsilane.

In the presence of a catalytic amount of active titanium(IV) salt generated in situ from 1 mol of TiCl4 and 2 mol of AgOTf, weakly nucleophilic anilines react under mild conditions with nearly equimolar amounts of silyl carboxylates to afford the corresponding anilides in excellent yields using 4-(trifluoromethyl)benzoic anhydride. The mixed anhydride formed in situ from trimethylsily acetate and 4-(trifluoxomethyl)benzoic anhydride, a key intermediate of this reaction, was detected by 1H NMR experiment. Further, it was shown that the reaction of the mixed anhydrides and 2-nitroaniline was faster than that of the corresponding homo anhydrides and 2-nitroaniline.

Catalytic Diels-Alder reactions between several 1,3-dienes and α,β-unsaturated ketones are effectively promoted by the combined use of diphenyltin sulfide (Ph2Sn=S) and silver perchlorate or Lawesson’s reagent (2,4-bis(4-methoxyphenyl)-1,3-dithia-2,4-diphosphetane-2,4-disulfide) and silver perchlorate under mild conditions.

1-t-Butyldimethylsiloxy-8,11,11-trimethylbicyclo[5.3.1]undec-7-en-9-one (2) has been synthesized via intramolecular aldol condensation of the precursor 3 by combined use of lithium diisopropylamide (LDA) and CeCl3. The dicarbonyl compound 3 was prepared by allylation of 8-membered ring ketone 6b.

Optically active 3,7-dibenzyloxy-4,8-di-t-butyldimethylsiloxy-5,5-dimethyl-6-p-methoxybenzyloxy-2-octanone (2) has been synthesized by way of diastereoselective aldol reaction between 4-benzyloxy-5-t-butyldimethylsiloxy-2,2-dimethyl-3-p-methoxy-benzyloxypentanal (3) and ketene silyl acetal 4 using MgBr2·OEt2 as a catalyst. The chiral pentanal 3 was synthesized either by asymmetric aldol reaction of both prochiral aldehyde 5 and ketene silyl acetal 4 using a chiral Lewis acid or by diastereoselective aldol reaction between the chiral aldehyde 6 derived from L-serine and the lithium enolate derived from methyl isobutyrate.

An efficient method for the synthesis of (−)-cephalosporolide D was established via successive enantioselective aldol reaction and effective construction of 8-membered ring lactone moiety.

Highly diastereoselective aldol reaction of α-bromo ketones with aliphatic aldehydes was successfully carried out by using titanium(II) chloride and copper in dichloromethane at low temperature. Similarly, Reformatsky-type reaction of α-bromo thioester with aliphatic aldehydes was promoted under mild conditions to afford β-hydroxy thioesters in good to moderate yields.

Various intermediates for the synthesis of erythronolide A, an aglycon of erythromycin A, are prepared from the corresponding seco-acids using 2-methyl-6-nitrobenzoic anhydride (MNBA) in the presence of 4-(dimethylamino)pyridine (DMAP) with or without triethylamine. The efficiency of the MNBA lactonization is assessed by studying this method and comparing the results with those of the other established macrocyclization protocols. It has been finally concluded that (i) the conformationally appropriate substrate for the monomeric cyclization gave the desired lactone in excellent yield under mild reaction conditions in the presence of MNBA and DMAP, (ii) the highly-strained substrate for the cyclization also afforded the monomeric lactone in relatively good yield at 100°C in toluene, and (iii) the seco-acid having stable linear conformation, which preferred dimerizing more than forming the monomeric lactone, provided the corresponding diolide in high yield with the constant ratio of the monomer to dimeric lactone (approximately 1/5). © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 305–320; 2009: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.200900017

Unsymmetrical alkyl 2-hydroxy-1-(1-hydroxyalkyl)alkyl ketones (bis-aldols) are successfully synthesized by way of sequential two aldol reactions using α-bromo ketones and two different aldehydes. In the first reaction, α-bromo-β-stannyloxy ketones are formed by tin(II) trifluoromethanesulfornate-promoted aldol reaction of α-bromo ketones with several aldehydes. Bis-aldols are then formed via the second aldol reaction between another aldehydes and dianionic enolates generated by the reduction of α-bromo-β-metalloxy ketones with titanium(IV) iodide and copper.

Various carboxylic esters are obtained at room temperature in excellent yields with high chemoselectivities from nearly equimolar amounts of carboxylic acids and alcohols using 2,6-dimethyl-4-nitrobenzoic anhydride with triethylamine by the promotion of 4-(dimethylamino)pyridine. The efficiency of the esterification is compared to those of other dehydrations using substituted benzoic anhydrides as coupling reagents. This method was successfully applied to the synthesis of threo-aleuritic acid lactone and the desired 17-membered ring compound was prepared in high yield at room temperature from the corresponding free trihydroxycarboxylic acid using 2,6-dimethyl-4-nitrobenzoic anhydride in the presence of 4-(dimethylamino)pyridine.

[Structure: see text] The total synthesis of pseudo 2-epibotcinolide (1b) through several featured synthetic approaches has been attained. First, the chiral linear precursors of the nine-membered ring compound is stereoselectively constructed by the asymmetric aldol reaction for producing beta-hydroxy ester units. Second, the key cyclization reaction to form the nine-membered lactone moiety is efficiently achieved by the extremely facile and powerful mixed-anhydride method promoted by 2-methyl-6-nitrobenzoic anhydride (MNBA) with basic promoters.

Various carboxylic esters are obtained at room temperature in excellent yields with high chemoselectivities from nearly equimolar amounts of carboxylic acids and alcohols using 2-methyl-6-nitrobenzoic anhydride with triethylamine by the promotion of a basic catalyst such as 4-(dimethylamino)pyridine. A variety of lactones are also prepared in high yields at room temperature from the corresponding omega-hydroxycarboxylic acids with use of 2-methyl-6-nitrobenzoic anhydride in the presence of 4-(dimethylamino)pyridine. A similar reaction occurs with triethylamine when using a catalytic amount of 4-(dimethylamino)pyridine 1-oxide as an effective promoter for the intramolecular condensation reaction. These methods are successfully applied to the synthesis of erythro-aleuritic acid lactone and an eight-membered-ring lactone moiety of octalactins A and B. The efficiency of the cyclizations is compared to those of other reported lactonizations.

An efficient mixed-anhydride method for the synthesis of carboxylic esters and lactones using benzoic anhydride having electron withdrawing substituent(s) is developed by the promotion of Lewis acid catalysts. In the presence of a catalytic amount of TiCl2(ClO4)2, various carboxylic esters are prepared in high yields through the formation of the corresponding mixed-anhydrides from 3,5-bis(trifluoromethyl)benzoic anhydride and carboxylic acids. The combined catalyst consisting of TiCl2(ClO4)2 together with chlorotrimethylsilane functions as an effective catalyst for the synthesis of carboxylic esters from free carboxylic acids and alcohols with 4-(trifluoromethyl)benzoic anhydride. Various macrolactones are prepared from the free ω-hydroxycarboxylic acids by the combined use of 4-(trifluoromethyl)benzoic anhydride and titanium(IV) catalysts together with chlorotrimethylsilane under mild reaction conditions. The lactonization of trimethylsilyl ω-(trimethylsiloxy)carboxylates using 4-(trifluoromethyl)benzoic anhydride is also promoted at room temperature in the presence of a catalytic amount of TiCl2(ClO4)2. An 8-membered ring lactone, a synthetic intermediate of cephalosporolide D, is successfully synthesized according to this mixed-anhydride method using 4-(trifluoromethyl)benzoic anhydride by the promotion of a catalytic amount of Hf(OTf)4.

A variety of lactones were prepared in high yields at room temperature from the corresponding ω-hydroxycarboxylic acids using 2-methyl-6-nitrobenzoic anhydride in the presence of 4-(dimethylamino)pyridine. A similar reaction also occurs with triethylamine when using a catalytic amount of 4-(dimethylamino)pyridine 1-oxide as an effective promoter for the intramolecular condensation reaction. These methods were successfully applied to the synthesis of erythro-aleuritic acid lactone and the efficiency of the cyclizations is compared to those of other reported mixed anhydride methods.

Highly diastereoselective aldol reactions of α-bromo ketones with several aldehydes were successfully carried out by using a combination of titanium(II) chloride and copper or sodium iodide in dichloromethane-pivalonitrile at low temperature. Similarly, Reformatsky reactions of α-bromo thioester with aliphatic aldehydes proceeded to afford β-hydroxy thioesters in good yields under mild conditions.

Unsymmetrical alkyl or aryl 2-hydroxy-1-(1-hydroxyalkyl)alkyl ketones (bis-aldols) were synthesized by the samarium(II) iodide-mediated aldol reaction of aldehydes with alkyl or aryl oxiranyl ketones. Bis-aldols were formed via the aldol reaction of aldehydes with samarium enolates generated by epoxy-fragmentation of oxiranyl ketones using two moles of samarium(II) iodide.

Carboxamides are prepared in high yields by dehydration condensation between nearly equimolar amounts of free carboxylic acids and amines both of which involve secondary or tertiary alkyl substituted ones with O,O'-di(2-pyridyl) thiocarbonate, a coupling reagent, in the presence of a catalytic amount of 4-(dimethylamino)pyridine.