Glycals, Diels-Alder reactions

In addition, iodine snccessfnlly catalyzed the electrophilic snbstitntion reaction of indoles with aldehydes and ketones to bis(indonyl)methanes [225], the deprotection of aromatic acetates [226], esterifications [227], transesterifications [227], the chemoselective thioacetalization of carbon functions [228], the addition of mercaptans to a,P-nnsatnrated carboxylic acids [229], the imino-Diels-Alder reaction [230], the synthesis of iV-Boc protected amines [231], the preparation of alkynyl sngars from D-glycals [232], the preparation of methyl bisnlfate [233], and the synthesis of P-acetamido ketones from aromatic aldehydes, enolizable ketones or ketoesters and acetonitrile [234],... 

B. H. Lipshutz, S. L. Nguyen, and T. R. Elworthy, Preparation and Diels-Alder reactions of a pyranoid vinyl glycal Model studies for anthraquinone aglycone and carbohydrate syntheses, Tetrahedron 44 3355 (1988). 

Ortho-thioquinones are also accessible by this route [238, 239]. They were treated with glycals as dienophiles with success (Table 5, entry 2). These reactions were designed [240] to offer an original glycosyl transfer, by achieving a Diels-Alder reaction with the appropriate a,adioxothione (Table 5, entry 4), and a simple Raney nickel reductive desulfurisation of the cycloadduct. 

In early work which have been summarized very recently [23], Danishefsky et al. have investigated hetero Diels-Alder reactions of carbonyl compounds in order to yield glycals. Numerous further contributions to the stereoselective synthesis of dihydropyran derivatives by high pressure or Lewis acid induced Diels-Alder reactions of carbonyl compounds have been made by Jurczak et al.,... 

Hetero-Diels-Alder reactions between various oxygen-substituted butadienes and formaldehyde In the presence of zinc chloride have been used to produce racemic glycal derivatives and hence D,L-pentoses (Scheme 1). ... 

Glycals can also undergo cycloaddition reactions and their derivatives are of interest for synthetic purposes. Diels-Alder [4+2], Paterno-Biichi [2+2], and 1,3-dipolar additions can be applied to the construction of fused cycloadducts (O Scheme 21) [26,27,28]. 

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