3,4-dihydro-2H-pyran → trans-6-Hydroxymethyl-2-methoxytetrahydropyran

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The absolute configuration for the product was determined by first isomerizing the dihydropyran derivative product in methanol by use of hydrochloric acid as a catalyst so that the product had a trans-isomer content of 95%, subsequently converting the ester into an alcohol in diethyl ether by use of lithium aluminum hydride, and then hydrogenating the double bonds by using platinum dioxide in methanol to give trans-6-hydroxymethyl-2-methoxytetrahydropyran, followed by optical rotation measurement. As a result, the optical rotation [α]20 of the above compound was found to be -119.9° (c=1.07, solvent: benzene). This found value was compared with the optical rotation [α]20 of +129.7° (c=4.3, solvent: benzene) for (2S,6S)-6-hydroxymethyl-2-methoxy-3,4,5,6-tetrahydropyran as described in J. Jurczak et al., J. Chem. Soc., Chem. Commun., pp. 540-542 (1983). Based on the comparison, the trans-isomer and cis-isomer were judged to be a (2R,6R)-isomer and a (2S,6R)-isomer, respectively.

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