Grignard reagents cyclic chelate model

Similar selectivities (>99 1) are observed in the cyclic chelation controlled reaction of a-benzyloxy carbonyls such as (4a equation 4) with Grignard reagents. However when the a-hydroxy group is protected as a silyl ether (4b), the selectivities observed in the addition reaction diminish (60 40), or reverse (10 90 Table 3). The nonchelating nature of a silyl group, as well as its steric bulk, are responsible for this change in selectivity. In the case of (4b), nucleophilic addition via the Felkin-Anh model effectively competes with the cyclic-chelation control mode of addition. 

A short synthesis of L-( — )-rhodinose (635), the trideoxyhexose subunit of the antibiotic streptolydigin, takes advantage of the propensity of Grignard reagents to add to lactaldehydes under chelation control (Cram s cyclic model) to produce 5y -diols. 

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