Glycosylidene Carbenes. Part 24 Reactivity modulation by protecting groups of the addition of glycosylidene carbenes to electron‐rich alkenes

Abstract

AbstractThe reactivity of glycosylidene carbenes derived from pivaloylated vs. benzylated diazirines 1 and 2 towards enol ethers have been examined. The pivaloylated 1 led to higher yields of spirocyclopropanes than the benzylated 2. Among the enol ethers tested, dihydrofuran 6 proved most reactive, yielding 71–72% of the spiro‐linked tetrahydrofuran 7, while the benzylated diazirine 2 afforded only 33% of the analogue 8 (Scheme 1 ). Other enol ethers proved much less reactive. The addition of 1 and 2 to the dihydropyran 10 and the 2, 3‐dihydro‐5‐methyl‐furan 15 gave low yields of single cyclopropanes (→ 12, 14, and16), and the glycals 17 and 18, and (E)‐1‐methoxy‐oct‐1‐ene (23) did not react. The main products of these reactions were the azines (Z, Z)‐ 11 and (Z, Z)/( E, E)‐13. Similarly, 1 and 2 reacted poorly with (Z)‐1‐methoxyoct‐1‐ene (24), leading to cyclopropanes 25/26/27 and 28/29/30/31 (Scheme 2). Main products were again the azines (Z, Z)‐11 and (Z, Z)/(E, E)‐13. The structure of 70 and 25 was established by X‐ray analysis (Figs. 1 and 2). The mechanism of addition of glycosylidene carbenes to enol ethers is discussed, AMI Calculations indicate that the LUMOcarbene/HOMOalkoxyalkene interaction is dominant at the beginning of the reaction, while the transition states are characterized by a dominant interaction of the doubly occupied, sp2‐hybridized orbital of the carbene with the LUMO of the enol ether. The relative reactivity of the carbenes towards either the enol ethers or the diazirines determine type and yields of the products.