Question

3) Suggest a multi-step sequence that allows for the conversion of the starting material to the corresponding product. &

Answer 1

HO 20. Sa **) (A). Reho : co (oxidation HO OH OH Rx MaBr ОН OR forward synthesis: acetaldehyle By Ви DHP, H Protecting group OH OTHP - Mg , dry ether ® DHP= dihydropyran DhPa dihydropyran = n MBO Bo hala 27 +HO 11 OTHP (Grignard reaction) OH Protection 87 By --074 group ex -HO OTHP THP= Tetrahydropyran ingbo Bringo H/H₂O -OTHP aceraldehyde) SOTHP OH Jones K₂Cr2O7, H250 4 Acid treatement (HD/H20) deprotects the THP group and gives - OH group back. oxidation reaction. )

Since Grignard reagent is basic, in presence of -OH group, which has acidic H, the reaction will not proceed as planned. Grignard reagent will abstract proton from another molecule and will get destroyed. That is why, -OH group needs to be protected. Here we choose the protecting group in such a way that it can also be deprotected easily (with acid treatment). So in one step (H+/H2O), both -OMgBr and -OTHP converts into -OH group.

All of the steps give very nice yields.