合成路线1

The reaction of the chiral diol (XXIV) first with MsCl and TEA, and then with NaCN gives the chiral butyronitrile (XXV), which is reduced with DIBAL in dichloromethane to yield the butyraldehyde (XXVI). The reaction of (XXVI) with allyl tributyl tin (XXVII), (S)-BINOL and Ti(OiPr)4 affords the secondary alcohol (XXVIII), which is condensed with acetal (XXIX) by means of PPTS to provide the diene (XXX). A ring closing metathesis reaction in (XXX) by means of a Ru catalyst gives the dihydropyran derivative (XXXI). Alternatively, the reaction of (XXVIII) with propenoyl chloride (XXXII) by means of TEA gives the diene (XXXIII), which is submitted to a ring closing metathesis reaction as before to yield the dihydropyranone (XXXIV). The reduction of (XXXIV) with DIBAL, followed by treatment with CSA and ethanol affords the dihydropyran derivative (XXXV). The reaction of both acetals (XXXI) or (XXXV) with CH2=CH-O-Tbdms (XXXVI) in the presence of Montmorillonite as Lewis acid, followed by a treatment with NaBH4 provides the dihydropyran (XXXVII) as a single isomer.