Bicyclic 1,3,2-oxazaborolidines

Bicyclic 1,3,2-oxazaborolidines synthest rymes. The crystalline BH3 adduct of the I... 

For the enantioselective reduction of cyclic meso-imides and 1 -siloxy-2-alkanones, 1,2-reduction of other enones,the prototypal chiral bicyclic oxazaborolidine is adequate, although 81 has been used to reduce a-phenylthio-p-phenyl ketones to obtain precursors of a-hydroxy thioesters. The enantiomer of 81 mediates reduction of enediones to the corresponding chiral diols. ... 

A disulfide-linked bis(aminoethanol) 82 prepared from L-cystine also catalyzes the borane reduction of ketones. Other oxazaborolidine derivatives are obtained from 83, 84, " and 85, " and polymer-bound species. Those derived from the ephedra bases find use in the asymmetric reduction of imines. bicyclic oxazaborolidine generated... 

A powerful catalyst for [2+2]cycloaddition is created from the bicyclic oxazaborolidine 135 and AlBr3. A hydrindanone frequently used in total synthesis of natural products is readily available from one such adduct in chiral form. ... 

An expedient and stereoselective synthesis of bicyclic ketone 30 exemplifies the utility and elegance of Corey s new catalytic system (see Scheme 8). Reaction of the (R)-tryptophan-derived oxazaboro-lidine 42 (5 mol %), 5-(benzyloxymethyl)-l,3-cyclopentadiene 26, and 2-bromoacrolein (43) at -78 °C in methylene chloride gives, after eight hours, diastereomeric adducts 44 in a yield of 83 % (95 5 exo.endo diastereoselectivity 96 4 enantioselectivity for the exo isomer). After reaction, the /V-tosyltryptophan can be recovered for reuse. The basic premise is that oxazaborolidine 42 induces the Diels-Alder reaction between intermediates 26 and 43 to proceed through a transition state geometry that maximizes attractive donor-acceptor interactions. Coordination of the dienophile at the face of boron that is cis to the 3-indolylmethyl substituent is thus favored.19d f Treatment of the 95 5 mixture of exo/endo diastereo-mers with 5 mol % aqueous AgNC>3 selectively converts the minor, but more reactive, endo aldehyde diastereomer into water-soluble... 

In the first step, borane coordinates to the nitrogen atom in oxazaborolidine 79b from the less hindered side of the fused bicyclic system, resulting in the... 

The reaction of diphenylketene with 1,3,2-diazaboroles 234 in hexane at — 20 °C leads to the formation of 1,3,2-oxazaborolidines 238 in 64-70% yield. The reaction proceeds via the coordination of the ketene oxygen to boron 236 followed by a [2+3] cycloaddition yielding the bicyclic intermediate 237. Fission of B-N bond leads to the product formation as yellow to colorless crystals (Scheme 40) <20000M5791>. 

Mc2S BHj in toluene or xylene at room temperature. Results from reduction of diethyl a-ketophosphates in the presence of the bicyclic l-butyl oxazaborolidine, and of numerous ketones using a monocyclic catalyst (17) are quite respectable. 

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