Miscellaneous cyclizations compounds

Diels-Alder Reactions of Vinylthiophenes and Related Compounds 2.11.6.5.1 Miscellaneous cyclizations... 

Abscisin II may be viewed as a monocyclic analog of these two anomalous sesquiterpenes. Alternatively, it may be one of a larger group of miscellaneous compounds which do not necessarily possess exactly 10 or 15 carbon atoms but may be looked upon formally (and perhaps actually) as degradation products of the carotenoids. This view is favored by the fact that the carotenoids are the only class of plant products in which cyclization of the type found in abscisin II is very common. Other examples of these compounds in-... 

Besides processes (1) and (2), the reader should be aware that nucleophilic attacks on alkynes are treated in other chapters of this book, dealing with rearrangements, cyclizations, polyacetylenes, cyclic acetylenes and perhaps others. A number of publications overlap with ours in different ways and at different levels -. They treat individual alkynes or families " , e.g. acetylene, diacetylenes , acetylene dicarboxylic esters haloacetylenes , alkynyl ethers and thioethers > ynamines , fluoro-alkynes ethynyl ketpnes , nitroalkynes , etc. synthetic targets, e.g. pyrazoles , if-l,2,3-triazoles , isothiazoles , indolizines S etc. reagents, e.g. nitrones , lithium aluminium hydride , heterocyclic A -oxides - , azomethine ylids - , tertiary phosphorus compounds , miscellaneous dipolar nucleophiles - , etc. The reader will appreciate that all of these constitute alternate entries into our subject. 

Miscellaneous. The phosphonium salt (92) has been shown to be an excellent reagent for the carboalkenylation of carbonyl compounds. For example, the sodium enolate (93) and (92) give a cyclic product (94) which is thought to arise from a stabilized ylide by cyclization via an intramolecular Wittig reaction. 

Miscellaneous Reactions of Oxirans.—The first successful enzymatic cyclization of a non-natural squalene has been disclosed. (18Z)-Oxidosqualene (188), which does not possess the naturally occurring a -trans stereochemistry, was caused to cyclize, in the presence of 2,3-epoxysqualene sterol cyclase, to (205)-epinorlanosterol (189). The polyene oxide (190) underwent an uncommon tricyclization in CH2CI2 containing Bp3-OEt2 to form the cw-fused A/B-ring 18-nor-steroid (191) (25%) this compound was found to be identical with a material derived by treatment of a naturally occurring steroid with BF3. 

As was the case for 5-exo-cyclizalions, aside from the more frequently encountered substructures that undergo 6-exo-cyclization, many miscellaneous examples can be found throughout the literature [118-120]. One study concerns the effect of the stereochemistry of vinyl bromo substrates 158 and 160 on the ratio of possible double-bond isomers in the product (Scheme 6.46) [121]. Starting from the cw-compound 158, bisunsaturated indolizidinone 159 was obtained along with only traces of the bis-exo-cyclic diene. The tran -compound 160 reacted twice as fast and gave a 4 1 mixture of 161 162. Product 161 is probably the first to be formed, and its more stable isomer 162 is most likely formed by a readdition and elimination of HPdBr. 

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