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Boron aldol additions of 1- O -silylated 3,4-di- O -benzyl- and 3,4-di- O -benzoyl- l -erythrulose and achiral aldehydes using dicyclohexylboron chloride have been investigated. The dibenzyl derivative gave syn/syn stereoisomers with high stereoselectivity, whereas the dibenzoyl derivative gave syn/anti stereoisomers. It is believed that, while the dibenzoyl erythrulose gives rise to the E enolate in the presence of dicyclohexylboron chloride, as usually observed with this reagent, only the Z enolate is formed in the case of the dibenzyl derivative.

We have investigated a number of nucleophilic additions of allylating reagents to several α,α′,β-trioxygenated ketones ( O -protected erythrulose derivatives). Reagents based on lithium, magnesium, copper and titanium gave low to medium stereoselectivities and did not display any recognizable trend in the sense of stereoselection. In contrast, reactions involving silicon and tin derivatives were highly stereoselective and gave rise to essentially a single diastereoisomer, the structure of which depended on the type of protecting group. Thus, α,β-di- O -benzylated derivatives experienced almost exclusive addition to the carbonyl Si side, whereas α,β- O , O -alkylidene derivatives (dioxolane acetals) yielded the opposite diastereoisomers as a result of addition to the Re side. These results suggest the intermediacy of α-chelates in the additions of silicon and tin reagents to the di- O -benzylated derivatives. In contrast, the opposite stereoisomers, formed in the reactions of dioxolanes, are believed to be formed through Felkin–Anh transition states, pointing again to the reluctance of acetal oxygens to participate in chelated intermediates.

The stereoselective synthesis of the naturally occurring lactones osmundalactone (−)-1 and muricatacin (−)-2 is described. The key steps in each synthesis are the stereoselective addition of a Grignard reagent to a suitably protected α-hydroxy aldehyde and a ring-closing metathesis. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

A short, stereoselective synthesis of the naturally occurring pyrrolidine radicamine B is reported. Garner′s (R)-aldehyde, prepared from d-serine, was the chiral starting material. The pyrrolidine ring was stereoselectively created in a very efficient way through a five-step, one-pot transformation. In addition, an intermediate of this synthesis was transformed into an intermediate of a previously published synthesis of the potent α-glucosidase inhibitor nectrisine.

A convergent stereoselective synthesis of the bacterial DNA primase inhibitor Sch 642305 and its non-natural epimer at C-4 is described. A key aspect was the construction of a trans-2,3-disubstituted cyclohexanone system by means of a stereoselective Michael addition/α-alkylation sequence. The macrolactone ring of either stereoisomer was created using the Mitsunobu and Yamaguchi procedures, respectively.

The published syntheses of naturally occurring 5,6-dihydropyran-2-ones are reviewed. Figure options Download full-size image Download as PowerPoint slide

The first total synthesis of the non-natural enantiomers of the fungal metabolites communiols A–C is reported. A stereochemical misassignment has been corrected and the absolute configurations of the natural products have been unambiguously established.

The structure reported in the literature for the pharmacologically active, natural lactone passifloricin A (δ-lactone of 2Z,5R*S*,7R*,9S*,11S*-tetrahydroxyhexacos-2-enoic acid) is incorrect not only as regards stereochemical issues, but also in the placement of one of the hydroxyl groups of the side chain. By means of an unambiguous synthesis, passifloricin A is shown to be the δ-lactone of 2Z,5R,7S,9S,12S-tetrahydroxyhexacos-2-enoic acid. All of the stereogenic centres were created with the aid of Brown's asymmetric allylation methodology. The lactone ring was made via ring-closing metathesis.

The first total synthesis of the naturally occurring lactone (+)-hyptolide is described. Ethyl l -lactate was the chiral starting material. Key steps of this 15-step synthesis were a Brown's asymmetric allylation, a Carreira's asymmetric ethynylation and a ring closing metathesis.

Boron enolates of an ethyl ketone structurally related to erythrulose react with achiral aldehydes in a highly stereoselective fashion to yield 1,2- syn /1,3- syn stereoisomers. Oxidative cleavage of the aldol adducts yields enantiopure O -formylated syn -α-methyl-β-hydroxy esters, easily cleaved to the corresponding hydroxyl-free compounds. The aforementioned ketone behaves therefore as a chiral propionate enolate equivalent.

A very short and stereoselective synthesis of the non-natural enantiomer of malyngolide from l -erythrulose is described. Key features of the synthesis are the Felkin–Anh diastereoselective allylation of a polyoxygenated ketone and the allylation/metathesis/allylic oxidation protocol recently described by our group.