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Our synthetic program (depicted in Scheme 1) starts from the readily available 4methyl phenacyl bromide 5, which on treatment with NaN3 in the presence of b-cyclodextrin11,12 in water at room temperature resulted in azide 6. This is identical in all respects with the reported one.13 Asymmetric reduction with azido aryl ketone 6–b-cyclodextrin complex and sodium borohydride12a in water produced 7 in 95% yield and 80% ee. The same ketone 6, when reduced with oxazaborolidine-catalyzed asymmetric borane14a yielded the alcohol 7 in 94% yield and 100% ee. There arecertain advantages with both the reagents as the earlier one is easily accessible, inexpensive, and recyclable but the lattergives good ee %. Alcohol 7 was protected as its MOM ether 8, which on oxidative cleavage with RuCl3 and NaIO4 gave methyl ester 9. Reduction of the azide group in 9 with b-cyclodextrin and TPP gave the corresponding amine, which on in situ protection with the Boc group produced 10.12e NaBH4 and LiCl mediated reduction15 of the methyl ester group in 10 resulted in alcohol 11. Alcohol 11 was oxidized under Swern conditions16 to give the corresponding aldehyde, which, as a crude,on Wittig olefination with MeTPPI and t-BuOK in THF at 0 C produced olefin 12. N-Allylation of 12 was achievedby treating with allyl bromide in the presence of NaH and a catalytic amount of TBAI to furnish 13, the precursor for the key reaction, ring closing metathesis (RCM).17 Substrate13 on treatment with Grubbs’ 1st generation catalyst in DCM produced 14 in 24 h at room temperature.Dihydroxylation9iof 14 with OsO4 and NMO in acetone and water medium gave 15, which on treatment with 6 M HCl in MeOH at room temperature resulted in the hydrochloride salt of piperidine derivative 1. Substrate 1 compared well with the reported data9 proving that the dihydroxylation of 15 was performed on the expected anti face. In another route, reduction of 14 using PtO2 under an H2 atmosphere resulted in piperidine derivative 16. Substrate 16 on treatment with 6 M HCl in methanol resulted in S-3-hydroxy piperidine, which without purification was subjected to Boc-protection in the presence of TEA and Boc-anhydride to furnish N-Boc piperidine 4, which was in good agreement with the reported data.18,9d |
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