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2-(o-Amidophenyl)oxa- and -thiazolines undergo excited-state intramolecular proton transfer (ESIPT), generating aza-o-xylylenes capable of intramolecular [4+2] and [4+4] cycloadditions with tethered unsaturated pendants. Facile hydrolysis of the primary photoproducts, spiro-oxazolidines and thiazolidines, under mild conditions unmasks a phenone functionality. Variations in linkers allow for access to diverse core scaffolds in the primary photoproducts, rendering the approach compatible with the philosophy of diversity-oriented synthesis. Chiral oxazolines, readily available from the corresponding amino alcohols, yield enantioenriched keto-polyheterocycles of complex topologies with enantiomeric excess values up to 90%.

Upon irradiation, cyclic imines containing o-amido groups are shown to produce reactive intermediates, amino azaxylylenes, which undergo intramolecular cycloadditions to tethered unsaturated pendants to yield complex N,O-heterocycles having an additional spiro-connected nitrogen heterocyclic moiety. Modular assembly of the photoprecursors allows expeditious increase of the complexity of the target poly-heterocyclic scaffolds with a minimal number of experimentally simple reaction steps. The photocyclization and subsequent postphotochemical transformations are accompanied by an increase of Lovering's fsp3 factor, thus producing unprecedented three-dimensional molecular architectures, and offering extended sampling of chemical space.

Aldingenin A is assigned the structure of 5-(S)-hydroxycaespitol (I) and aldingenin B the structure of hemiacetal (II).

Fused hydantoins [(V) and (VI)], oxazolidinones [(X), (XIV), and (XV)], and imidazolines [(XIX) and (XX)] are synthesized through photoinduced intramolecular cycloadditions of appropriate precursors containing acylurea (IV), urethane [(IX) or (XIII), resp.], or urea (XVIII) linkers, respectively.

The photogenerated diketopiperazinoquinolinol products (II) contain a reactive oxirane moiety which proves synthetically useful for further transformations.

The modular synthesis of photoprecursors and their photoinduced cyclization into substituted 1-benzazocanes of two distinct topologies is described. The key step producing an extended polyheterocyclic system involves the photogeneration of azaxylylenes and their subsequent intramolecular cycloaddition with furan-containing pendants tethered either via the aniline nitrogen or through the carbonyl group containing arm. The primary photoproducts—secondary or tertiary anilines which are not acylated at the nitrogen atom—undergo facile acid-catalyzed or spontaneous ring-opening–ring-closing rearrangement to yield fused polyheterocyclic structures possessing a 2,6-epoxyazocane (or oxamorphan) core.

Modular preassembly of azaxylylene photoprecursors, halogen-substituted in the aromatic ring, their intramolecular [4 + 4] or [4 + 2] cycloadditions to tethered unsaturated pendants, and subsequent postphotochemical capstone modification of the primary photoproducts via Suzuki coupling provides rapid access to diverse biaryls of unprecedented topology. This synthetic sequence allows for rapid growth of molecular complexity and is well aligned with methodology of Diversity-Oriented Synthesis.

A variety of cyclic N-sulfonyl amidines is constructed through the photoinduced cyclization of the N-tethered pyrroles to azaxylenes.

Novel photocycloaddition reactions of o-azaxylylenes, generated by excited-state intramolecular proton transfer from o-amidoaryl aldehydes and ketones, result in the stereoselective formation of polycyclic [4 + 2] and [4 + 4] adducts.

The present review deals with the application of sulfenic [RS-OH] and sulfoxylic [HO-S-OH] acid derivatives in electrophilic addition reactions. We consider a new method for generation of reactive electrophilic sulfenylating species from sulfenic/sulfoxylic acid derivatives and sulfur trioxide (or its complexes). This autoreview covers our results in the systematic study of SO3-mediated addition to olefins and of sulfenylation of olefins with these novel electrophiles. The reactions of disulfides, sulfenyl chlorides, sulfenamides, alkyl sulfenates, aminosulfenyl halides and thiobisamines with a wide range of unsaturated compounds are discussed. A special chapter concerns the acid-catalyzed nucleophilic substitution of the products obtained.

Areneselenenyl trifluoroacetates and triflates were obtained by comproportionation of an areneseleninic anhydride with the corresponding diselenide in the presence of trifluoroacetic or triflic anhydride. This represents an attractive alternative to the previously reported1–5 route for preparation of these reagents.

The present review covers basic work on the synthesis, chemical reactions and practical application of organic thiosulfonates published before 1969, and gives more detailed attention to papers which appeared in the literature during the last 20 years. Three approaches to the synthesis of thiosulfonates, namely, (i) oxidation-reduction techniques, (ii) nucleophilic and, (iii) electrophilic introduction of an RSO2S moiety into an organic substrate, are presented in this review. (The synthetic part is divided into two subchapters: the synthesis of “symmetric” thiosulfonates and of “unsymmetric” ones). Nucleophilic substitution, electrophilic reactions, thermolysis and photolysis of thiosulfonates are also described. A special chapter serves as a brief summary of some practical applications of thiosulfonates.

An efficient tin-free reductive photochemical carboxymethylation of olefins with α-alkylthioacetates is developed, which (a) is an experimentally simple technique furnishing substituted derivatives of fatty acids in moderate to good yields, (b) is compatible with polar/protic functional groups and solvents, (c) does not require a special reducing reagent, (d) is fairly insensitive to the presence of dissolved oxygen and, generally, does not require degassing prior to irradiation. The critical feature of the mechanism in the “reluctance” of short-lived MeS • to group-transfer, resulting in hydrogen abstraction from the media.

Nitrosonium ethyl sulfate (1), which is generated in situ from ethyl nitrite and sulfur trioxide, is a convenient reagent for the one-pot transformation of olefins and dienes into substituted aldeh...

The condensation of dehydroacetic acid with RFCO2Et in the presence of LiH in THF afforded fluorine-containing 3-acetoacetyl-4-hydroxy-6-methylpyrones, which underwent cyclization to 2-polyfluoroalkyl-7-methylpyrano[4,3-b]pyran-4,5-diones on treatment with concentrated sulfuric acid at room temperature.

Page 20 Volume 17 Issue 4 2004 The Spectrum organic synthetic sequences, which makes assembly very effi cient and straightforward. Second, the nature of C-C bond allows for building robust molecular systems capable of withstanding drastic conditions. And fi nally, the photocleavage can be driven to completion; the release of the carbonyl compounds is nearly quantitative, especially in the case of aromatic aldehydes and ketones. The absolute quantum yields vary from system to system and reach as high as 56% for bis-methyldithiane adducts in polar acetonitrile. Initial studies of the photoinduced fragmentation of 2(α-hydroxy-benzyl)-1,3-dithianes utilized classical physical organic methods such as the Hammett substituent effect, the kinetic isotope effect,2 and a laser fl ash photolysis study.3 These results are in agreement with a mechanism, similar to the Grob-like C-C bond cleavage of 2-aminoalcohols described earlier.4