Synthesis of Optically Active A-Ring Fragments of Taxol via Electrophilic Ring Closure of an Epoxy-Allylsilane.

Cyclizations versus rearrangements in the reactions of some epoxyolefins with Lewis acids†

“…This substrate gave a quite high yield of the desired cyclohexane derivative 2 (Scheme 1) without any traces of the rearranged product. 21,22 We assume that in this case the hydroxy did not react with BF 3 ؒOEt 2 to give a borate but perhaps only gave a coordination complex. Thus, the BF 3 -coordinated HOCH 2 -group (BF 3 ؒHOCH 2 -) seemed to be less prone to migrate than TBDMSOCH 2 -.…”

“…Since 18, 22 carrying an allylic alcohol moiety as an alternative starter group, was the starting material for 1 it was natural to test 18 for cyclization (Scheme 6). Being a primary allylic alcohol it would have a tendency to rearrange to the tertiary allylic alcohol 18a possibly via cation 19 or an equivalent species.…”

“…The common starting material for the allylsilanes was the earlier described doubly protected isopropyl ester 41 22 In the Ni-catalyzed coupling of 47a and 48a with TMSCH 2 -MgCl both compounds gave, in addition to the expected allylic silanes 50a and 51a, respectively, the corresponding minor coupling products 50b (8%) and 51b (14%), resulting from coupling at the allylic THP ether site (Felkin-type coupling 64 ). Since the desired coupling reaction was favoured by the use of an excess of the Grignard reagent a compromise was to use 2 equivalents of this reagent, which, still gave some Felkincoupling.…”

“…[10][11][12][13][14][15][16][17][18][19][20] During our previous work with the synthesis of taxol A-ring building units we noticed that the diastereomeric epoxyallylsilanes 1 and 3 gave completely different products on treatment with BF 3 ؒOEt 2 . 21, 22 While 1 gave the cyclohexane derivative 2 (Scheme 1), compound 3 with the other configuration at the epoxide unit produced the rearranged product 4 and fluorohydrin 5 (Scheme 1). Related examples of diastereoselective ring closure of substituted epoxyallylsilane have been reported.…”

Stereoselective and Regioselective Preparation of C‐Pentopyranosides and Formal Synthesis of Omarigliptin

“…However, in the cases of C ‐pentopyranoside epoxides 17 and 20 , a simple lithium aluminum hydride (LAH) reduction in THF showed complete regioselectivity and led to high yields of 3‐deoxy C ‐pentopyranosides 18 and 21 , respectively. More interestingly, it was even possible to oxidize the free hydroxy group in compound 20 and then open the epoxide ring under very mild conditions to give ketone 23 in 89 % yield. In this way, the 2,4‐dihydroxy groups in C ‐pentopyranoside 21 were efficiently differentiated (Scheme ).…”