Dehydrogenation with selenium

Hi) Dehydrogenation. j3-Carboline derivatives may be obtained from tetrahydro-)3-carbohnes by zinc dust distillation or high temperatmre dehydrogenation with selenium or palladium black. Many of the complex indole alkaloids may be degraded, with bond cleavage, to yield simple )3-carbolines under these conditions and this approach has become a standard method in structural elucidations. Examples are numerous but outside the scope of this review. 

Dehydrogenation with selenium dioxide gives the l,4-dien-3-one (19-1). The great majority of such glycols are commonly used as their 16,17-cyclic acetals with some small ketone. In the case at hand, reaction with acetone gives the acetal 19-2, that group often being called an acetonide. 

When cryptolepine is dehydrogenated with selenium there is formed a base, C15H10N2 (m.p. 249-251°), which on treatment with methyl iodide forms a methiodide which is identical with cryptolepine hydriodide. When distilled with soda lime cryptolepine generates a base, Ci6Hi2N2 (m.p. 264-265°), isomeric with its parent, but unlike its parent it now has a C-methyl group indicating a shift from nitrogen to the nucleus. The spectra of these and other bases and derivatives indicated that cryptolepine should be the anhydronium base derived from quindoline methiodide (32-34) and that the selenium treatment base should be quindoline (X). Cryptolepine therefore can be represented by XI, which is the structure most probable in an alkaline environment. 

It has been observed that all the steroids on dehydrogenation with selenium at 360°C usually yield DieTs hydrocarbon, I. e., 3 -methyl-l 2-cyclopentanophenanthrene, whereas at 420°C, the steroids give mainly chrysene and a small amount of picene. 

DEHYDROGENATION. The definite determination of the skeletal structure of solanidine, and particularly that portion of the molecule which carries the nitrogen, was accomplished by dehydrogenation with selenium and identification of the products. Early attempts in this direction led to phenanthrene, chrysene, and pyridine (23) presumably because of too drastic conditions. Under so called normal conditions, that is tempera-... 

Upon dehydrogenation with selenium protoveratrine has yielded... 

Isolated from American colophony, (rosin). Plates. M.p. 170-4°. [a]n — 102° in EtOH. Sol. MeOH, EtOH, Et,0, AcOH, Me CO, CHClj, CfHg. Insol. HgO. Forms additive compound with maleic anhydride. Dehydrogenation with selenium —>> retene. Absorption maximum at 237-5 mu. 

The second group of workers (16) obtained 1,2,8,10-tetramethyl-phenanthrene (IX) from the diketone VI-B by the reaction with methyl-lithium followed by a dehydrogenation with selenium. Compound VI-B was prepared from C-3 desoxycassenic acid obtained from a Clemmensen reduction of diketocassenic acid. Further, the acetoxy diketone VI-A was converted to 1,2,8-trimethylphenanthrene by a Wolff-Kishner reduction and mild chromic acid oxidation followed by the Grignard and dehydrogenation sequence. 

Harman and brevicarine, C17H21N3 (mp 104°) B-2HC1 (mp 189°). The latter is an indole derivative (27). Brevicolline on dehydrogenation with selenium yields a number of products containing the indole nucleus. A reactive methyl group is indicated by the fact that the alkaloid condenses with benzaldehyde. Structure XI is tentatively suggested (28, 29). 

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