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Oxazoles have been shown to behave as dependable azadiene components in Diels-Alder reactions.1) Since Kondrat'eva reported the first example of a Diels-Alder reaction of an oxazole with an olefin to produce a pyridine in 1957, 2,3) this methodology has become a valuable tool for the preparation of highly substituted pyridines, such as pyridoxine and its analogues. Despite an early recognition of the practical value of the oxazole-olefin Diels-Alder reaction, there are few reports applying this cycloaddition intramolecularly to the synthesis of pyridine-containing natural products. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In the present study, we sought to explore the feasibility of intramolecular oxazole-olefin Diels-Alder reaction for an efficient construction of two monoterpene alkaloids possessing the cyclopenta[c]pyridine ring system. [20][21][22][23] Plectrodorine (1), selected as the first target for the monoterpene alkaloids, was isolated as a racemate together with isoplectrodorine (2) from the aerial parts of Plectronia odorata (Rubiaceae) by Koch and co-workers.24) The structure and relative stereochemistry of 1 were elucidated through a combination of spectral analysis and chemical transformation. The Koch group 25) then described the isolation of oxerine from the aerial parts of Oxera morieri (Verbenaceae) and proposed its absolute stereochemistry to be (5R,7S)-3 by partial synthesis of oxerine from harpagide of known absolute configuration. 26) We chose (5R,7S)-3 as the second target to demonstrate the versatility of our synthetic strategy, although racemic synthesis of oxerine has been accomplished by several research groups. [27][28][29][30] A brief account of the results reported here has been published in a preliminary form. 31)For the construction of the cyclopenta[c]pyridine skeleton via the intramolecular Diels-Alder reaction of oxazoles, we planned to employ the oxazole-olefin 4. The introduction of suitable olefinic dienophiles to the oxazole aldehyde 5 would provide 4, whereas the oxazole ring of 5 was envisaged to arise from the addition of a-lithiated methyl isocyanide to the g-butyrolactone 6 according to the procedure of Jacobi. 32,33) The requisite g-butyrolactone 6 was readily obtained from Seebach's dioxolanone 7. 34,35) Thus, reduction of 7 with BH 3 · Me 2 S followed by alkaline hydrolysis and acid-promoted lactone formation afforded 6 in 73% yield. The absolute configuration of 6 was further substantiated by the identity of specific rotation of the benzoate 8, derived from 6, with the data reported in the literature. 36) After protection of the tertiary hydroxy group in 6 with tert-butyldimethylsilyl triflate (TBDMSOTf) to afford the lactone 9, the formation of an oxazole ring was carried out by treatment of 9 with 2.5 eq of a-lithiated methyl isocyanide in THF at Ϫ78°C for 3 h followed by addition of AcOH, a slight modification of the Jacobi method, 32,33) giving the alcohol 10 in 66% yield. With a view to introducing an olefinic dienophile, the alcohol 10 was...

Two new copper(II) complexes with imidazole-4-carboxylate and imidazole-4-dithiocarboxylate have been synthesized. The monomeric units are connected by NH···O=C and NH···S=C intermolecular hydrogen bonds, respectively, yielding one-dimensional chains that stack with unusually short π–π distances.

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