Efficient “Dehomologation” of Di-O-isopropylidenehexofuranose Derivatives To Give O-Isopropylidenepentofuranoses by Sequential Treatment with Periodic Acid in Ethyl Acetate and Sodium Borohydride

Total Synthesis of Brevetoxin A: Part 1: First Generation Strategy and Construction of BCD Ring System

“…[47] d-Glucose (5) was expediently converted to bis(acetonide) 41 (Scheme 7) by a known procedure [32a] and thence selectively cleaved to aldehyde 42 by H 5 IO 6 . [48] Addition of MeMgBr to 42 (75 % overall yield from 41), followed by Swern oxidation [49] [(COCl) 2 /DMSO, Et 3 N, 80 % yield] afforded methyl ketone 44 via alcohol 43 (mixture of diastereoisomers). Treatment of 44 with the reagent generated by mixing of allylmagnesium bromide with Ti(iPrO) 4 in THF at À 78 8C furnished tertiary alcohol 45 as a single stereoisomer in 94 % yield (nonchelation-controlled addition).…”

Larger laboratory scale synthesis of 5-methyluridine and formal synthesis of its L-enantiomer

“…Due to small difference of the Rf values of 27 and 28, their separation was successful at small scale only (ca 2 g of mixture per run). The 1,2;5,6-di-O-isopropylidene-α-L-glucofuranose 27 was then subjected to the oxidation/reduction (a. CrO3-Py-Ac2O; 34 b. NaBH4) sequence to invert the configuration at the C3 position to produce the L-allose 29 which in turn was subjected to dehomologation (a. H5IO6, b. NaBH4 36,37 ) at a 1 g scale which furnished the 1,2-O-isopropylidene-α-L-ribofuranose 32 in a cumulative 69% yield (29→ 32). As mentioned above for the D-enantiomer this process did not function well at elevated scale.…”

“…a transformation of the D-allose (a hexose) to D-ribose (a pentose) can be performed using orthoperiodic acid H5IO6 followed by NaBH4 reduction. 36,37 The orthoperiodic acid is strong enough (pKa ca 3.3 38 ) to promote a hydrolysis of the more reactive C5-C6 acetonide in 9 to liberate a diol 10 which was subsequently cleaved by the same reagent to furnish the aldehyde 11 which in turn was subjected to NaBH4 reduction to yield the ribo compound 12. This procedure is very attractive and in fact it was successfully used on a small scale (1 g) in good yields (e.g.…”

Total Synthesis of Marine Natural Products: Cephalosporolides

“…The diacetonide 74 was benzylated to 121 and further converted through Robins's dehomologation protocol [59] and acylation to 122 (Scheme 21). Allylation of 122 gave 123 and further acetyl group removal, TBS protection and mesylation provided 124.…”