Diels-Alder reactions arylboronic acids

Chiral boron(III) Lewis acid catalysts have also been used for enantioselective cycloaddition reactions of carbonyl compounds [17]. The chiral acyloxylborane catalysts 9a-9d, which are also efficient catalysts for asymmetric Diels-Alder reactions [17, 18], can also catalyze highly enantioselective cycloaddition reactions of aldehydes with activated dienes. The arylboron catalysts 9b-9c which are air- and moisture-stable have been shown by Yamamoto et al. to induce excellent chiral induction in the cycloaddition reaction between, e.g., benzaldehyde and Danishefsky s dienes such as 2b with up to 95% yield and 97% ee of the cycloaddition product CIS-3b (Scheme 4.9) [17]. 

A better accessible chiral mediator is the (acyloxy)borane (CAB) 2-64 prepared in situ from a tartaric acid derivative and arylboronic acid at room temperature. Hetero Diels-Alder reaction of benzaldehyde and Danishefsky s diene 2-10 in the presence of 2-64 gave the corresponding pyrone after acidic work up with 52 - 95 % ee depending on R. The best results were obtained with R = 2,4,6-Me3Ph and 2,4,6-iPr3Ph. Similarly, with 2-60 the pyrone 2-61 with up to 97% ee was found [107]. 

We have developed a stable CAB 2 (R = aryl) complex that can be prepared in situ by mixing tartaric acid derivative and arylboronic acid at room temperature. In contrast with 2, R = H, which is both air- and moisture-sensitive, the B-alkylated catalyst 2, R = aryl or alkyl, is stable and can be stored in closed containers at room temperature (Eq. 39). A solution of the catalyst (20 mol %) is effective in catalyzing the hetero Diels-Alder reaction of aldehydes with a Danishefsky diene to produce dihydro-pyrone derivatives of high optical purity (up to 98 % ee) (Eq. 40) [39]. The extent of asymmetric induction is largely dependent on the structure of the boronic acid. In general, bulky phenylboronic acid (R = 2,4,6-Me3CeH2, o-MeOC6H4) results in excellent asymmetric induction. 

He and his students developed C-alkylation with quaternary ammonium salts and nucleophilic displacements on such salts, including the stereochemistry. His name is immediately associated with important innovations in the use of polyphosphoric acid for inter- and intramolecular condensations, cyclizations, and functional conversions in organic chemistry. He pioneered the use of boron trifluoride as an efficient catalyst in the Fischer indole synthesis and discovered new reactions of anils, including Diels-Alder reactions. He and his students delineated the requirements for disproportionation of tertiary amines. He developed the synthesis and chemistry of arylboronic acids. One of his fundamental ideas was for the incorporation of sufficient boron into organ-specific drugs that they could then be... 

Since our group (22) and Hehnchen s (23) independently announced a new class of chiral acyloxyboranes derive from iV-sulfonylamino acids and borane THF, chiral 1,3 -oxazaborolidines, their utility as chiral Lewis acid catalysts in enantioselective synthesis has been convincingly demonstrated (2(5). In particular, Corey s tryptophan-derived chiral oxazaborolidines 10a and 10b are highly effective for not only Mukaiyama aldol reactions (24) but also Diels-Alder reactions (25). More than 20 mol% of 10b is required for the former reaction, however. Actually, the reaction of the trimethylsilyl enol ether derived from cyclopentanone with benzaldehyde afforded the aldoI products in only 71% yield even in the presence of 40 mol%of 10b (24). We recently succeed in renewing 10b as a new and extremely active catalyst lOd using arylboron dichlorides as Lewis acid components (2(5). 

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