Syntheses with Miscellaneous Diels-Alder Reactions

Syntheses with Miscellanous Hetero Diels-Alder Reactions 

The outstanding versatility of 1,2-oxazine derivatives resulting from hetero Diels-Alder reactions of nitroso dienophiles has been exploited in a multitude of natural product syntheses which have been reviewed recently [8], Therefore the discussion in this paper shall focus on some very recent, typical applications of nitroso dienophiles. 

An asymmetric Diels-Alder reaction of a chiral nitroso dienophile has been employed by Ganem et al. in order to open an elegant access to enantiomerical-ly pure (-t-)-mannostatin A 7-57 and several derivatives thereof [519], The cycloaddition of the heterodienophile 7-54 derived from mandelic acid to 1-me-thylthiocyclopentadiene 7-55 proceeded only in moderate diastereoselectivity, however, the desired product 7-56 was easily separated from its diastereomer. 

It is noteworthy that both components are highly reactive and both required an in situ generation. The cycloadduct 7-56 was then subjected to a reductive ring contraction, acetylation, and a highly diastereoselective bishydroxylation to yield 7-58 which upon several protecting group transformations gave man-nostatin A 7-57 in enantiomerically pure form (Fig. 7-13). 

Many further important natural product syntheses are covered by the aforementioned review. Particularly noteworthy amongst them are Hudlicky s syntheses of conduramines [522] and (+)-lycoricidine [523] since they employ enantioselective microbial oxidations of halobenzenes as source of chirality. Racemic lycoricidine has also been prepared by Martin et al. this synthesis exhibits an interesting Heck cyclisation as key step in addition to the hetero Diels- 

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