Methyl 3,4-0-benzylidene selective acylation

Ethoxycarbonyl chloride is, by virtue of resonance involving the ester function, less reactive than acetyl chloride, and the reagent has found application for selective O-acylation in the steroid field.34 With this reagent, methyl 4,6-O-benzylidene-a-D-glucopyranoside yielded35 2- and 3-esters in the ratio 24 1, and the related benzyl-thiocarbonyl chloride gave the 2-ester in 58% yield.36... 

An interesting observation is that the stereochemistry of the anomeric center can greatly influence the selectivity of acylations [70]. For instance [70b], the acetylation of methyl 4,6-0-benzylidene-a-D-glucopyranoside with lipase from Pseudomonas fluorescens yielded the 2-0-acetyl derivative exclusively, isolated in 94% yield. The fi anomer, however, acetylated under the same conditions, gave mainly the 3-0-acetyl derivative, isolated in 86% yield. 

Radical methylation of pyridazine and 3- or 4-methylpyridazine occurs preferentially at the 4,5-positions, but selectivity is less marked than for radical acylation. Thus all of the possible methylation products were observed and preparative isolation of the individual products, which were identified by GC-MS, was not possible (Scheme 27). Similarly, the major product from ethyl pyridazine-4-carboxylate, the 5-methyl derivative, could not be purified, but was characterized as readily isolable benzylidene derivatives <84H(22)1395>. 

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