Ketone, cyclopentenyl methyl

The ketone IR absorption of 3-methyl-2-cyclohexenone occurs near 1690 cm-1 because the double bond is next to the ketone group. The ketone IR absorption of 3-cyclopentenyl methyl ketone occurs near 1715 cm-1, the usual position for ketone absorption. 

As shown for the transformations of the cyclopentenyl methyl ketones (3a-d) to mixtures of (4a-d) and (5a-d) (Scheme 1), the ODPM rearrangement occurs from the lowest electronically excited p,y-enone triplet state. - The ir. ir configuration has been assigned to this state on the basis of CNDO-MO calculations, phosphorescence studies at 77 and mechanistic examinations. The phosphorescence data together with sensitization experiments have revealed a Ti energy range of 289-310 kJ mol (4.18 kJ = 1.0 kcal) for (3a-c) and a much lower Ti of 253 kJ mol for (3d). ... 

Lalande and coworkers have studied the regiochemistry of cyclization of keto aldehyde (44 equation 107). In this system, aqueous KOH gives exclusively the cyclopentenyl methyl ketone (45), while piperidinium acetate leads to cyclopentenecarbaldehyde (46). Similar results were obtained by Wolinsky with the isopropenyl analog of (44). These examples also demonstrate the preference for formation of a five- over a seven-membered ring. 

The aldol condensation also offers a convenient way to synthesize molecules with five- and six-membered rings (and sometimes even larger rings). This can be done by an intramolecular aldol condensation using a dialdehyde, a keto aldehyde, or a diketone as the substrate. For example, the following keto aldehyde cyclizes to yield 1-cyclopentenyl methyl ketone ... 

Henne, A., Siew, N.P.Y., and Schaffner, K., SI vs T2 photoreactivity of P,y-unsaturated ketones. Temperature dependant photo-ClDNP of 2-cyclopentenyl methyl ketones,/. Am. Chem. Soc., 101, 3671, 1979. 

Gonzenbach, H.-U, Tegmo-Larsson, l.-M., Grosclaude, J.-R, and Schaffner, K., The photochemistry of 2-cyclopentenyl methyl ketones, Helv. Chim. Acta, 60,1091,1977. 

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