Oxazaborolidinium salts

The preceding schemes for preparing estrone have the disadvantage that they yield the steroid as a mixture of the two enantiomers. Preparing material identical with that which occurs in Nature requires resolution of those diastereomers. This also implies loss of half of the mass of final product. A synthesis that produces estrone and its derivatives directly without the need for that extra step depends on the use of chiral auxiliaries in the formation of ring C. The crucial step in this synthesis involves Diels-Alder condensation of the diene 16-1 with the fumaric ester aldehyde 16-2 in the presence of the oxazaborolidinium salt shown in Figure 3.1 this reaction affords the tricyclic intermediate 16-3 as a single enantiomer (Scheme 3.16). 

Alkylation of the metallated enol derived from 52 with m-methoxy-phenylethyl-iodide to afford the tricyclic jj-keto ester 53, followed by cationic cydization of this to furnish the steroid derivative 54, warrants particular attention. Corey and colleagues have recently published another total synthesis of 40 [82], beginning with an enantioselective Diels-Alder reaction between Dane s diene 14 and dienophile 61. An oxazaborolidinium salt (see Section 1.3.2.3) was used as an efficient catalyst (Scheme 1-14). 

Cyclic enones (221) react with diazoacetates (222) in the presence of the Lewis acidic BF3 or oxazaborolidinium salt (223) to afford -functionalized enones (224) as a first example of the catalytic C -H functionalization. ... 

Chiral oxazaborolidinium ion is an excellent catalyst for the cyanosilylation of methyl ketones promoted by cyanotrimethylsilane and diphenylmethyl phosphine oxide (eq 27). Chiral thiourea (eq 28) and amino acid salt (eq 29) have been developed and employed as catalysts in the enantioselective s)m-thesis of cyanohydrins and their derivatives. 

Ryu and coworkers reported a highly enantioselective and (Z)-stereocontrolled three-component coupling reaction of a,P-acetylenic esters, aldehydes, and tri-methylsilyl iodide (TMSI) using chiral cationic oxazaborolidinium catalysts (Scheme 31.42) [54]. Both enantiomers of (Z)-P-iodo MBH esters (R/S) could be obtained enantioselectively by using an (S)- or (R)-oxazaboroUdinium salts (143 or 144), which behave as chiral Lewis acids and have been proven to be effective catalysts for Diels-Alder reactions, cyanosilylations, and Michael reactions. These esters can be directly converted into the optically active (Z)-P-branched derivatives... 

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