title: An enantioselective synthetic route toward second-generation light-driven rotary molecular motors.
abstract: Controlling the unidirectional rotary process of second-generation molecular motors demands access to these motors in their enantiomerically pure form. In this paper, we describe an enantioselective route to three new second-generation light-driven molecular motors. Their synthesis starts with the preparation of an optically active alpha-methoxy-substituted upper-half ketone involving an enzymatic resolution. The subsequent conversion of this ketone to the corresponding hydrazone by treatment with hydrazine led to full racemization. However, conversion to a TBDMS-protected hydrazone by treatment with bis-TBDMS hydrazine, prepared according to a new procedure, proceeds with nearly full retention of the stereochemical integrity. Oxidation of the TBDMS-protected hydrazone and subsequent coupling to a lower-half thioketone followed by recrystallization provided the molecular motors with >99% ee. As these are the first molecular motors that have a methoxy substituent at the stereogenic center, the photochemical and thermal isomerization steps involved in the rotary cycle of one of these new molecules were studied in detail with various spectroscopic techniques.
authors.affiliation: Stratingh Institute for Chemistry, Zernike Institute for Advanced Materials, and Center for Systems Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands.
authors.forename: Thomas C
authors.initials: TC
authors.lastname: Pijper
authors.forename: Dirk
authors.initials: D
authors.lastname: Pijper
authors.forename: Michael M
authors.initials: MM
authors.lastname: Pollard
authors.forename: Frédéric
authors.initials: F
authors.lastname: Dumur
authors.forename: Stephen G
authors.initials: SG
authors.lastname: Davey
authors.forename: Auke
authors.initials: A
authors.lastname: Meetsma
authors.forename: Ben L
authors.initials: BL
authors.lastname: Feringa