9.10- Dihydrophenanthrene dehydrogenation

The 1H—NMR spectrum of 5,6,17,18-tetrahydro[2.2](2,7)phenan-threnopane (48) is compatible with the dihydrophenanthrene units being in an anti position. Dehydrogenation of 48 with 2,3-dichloro-5,6-dicyano-p-benzoquinone in benzene gave [2.2](2,7)phenanthrenophane which, owing to the similarity of its 1H—NMR spectrum to that of 2,7-dimethyl-phenanthrene, is assumed to have its phenanthrene units in the anti position 71>. 

The majority of the 4a,4b-dihydrophenanthrenes listed in Tables 1-9 are definitely thermally unstable at room temperature or below and undergo rapid dehydrogenation by molecular oxygen according to the overall process D. ... 

The synthesis of 9-phenylphenanthrene (Expt 6.14) illustrates the formation of the phenanthrene system by the cyclisation of a 1,2-diphenylethylene (stilbene). The process involves an allowed photochemical cyclisation which gives initially a dihydrophenanthrene. This is readily dehydrogenated in situ by molecular oxygen in the presence of iodine. 

There are two types of enamides readily available for the simple synthesis of protoberberine alkaloids via the route involving photocyclization. One is the 2-acyl-1-benzylideneisoqui noline-type enamide that has a stilbene structure in the molecule and is known to undergo a six -electron conrota-tory electrocyclic cyclization to a dihydrophenanthrene and subsequent dehydrogenation (103), and thus provides a useful synthetic route to apor-phine alkaloids. The second is the 2-aroyl-l-methyleneisoqui noline-type enamide that gives the 8-oxoberbine in good yields on irradiation. 

Photochemical conversion of stilbenes to phenanthrenes via a six 7t-elec-tron conrotatory cyclization according to an electrocyclic mechanism to the dihydrophenanthrenes and subsequent dehydrogenation is a very famous and useful synthetic reaction (103). 

A Convenient method for obtaining pure 2-acylphenanthrene is the acylation of 9,10-dihydrophenanthrene followed by sulfur dehydrogenation. In this case, only the 2-position is attacked the over-all yield is about 48%. Anthracene is acylated in the 9-position (60%). The isomeric acetylacenaphthenes have been prepared from the hydrocarbon and acetic... 

The nuclear amino group is stable during the sulfur dehydrogenation of 2-amino-9,10-dihydrophenanthrene (cf. method 2). ° In another instance, it is protected by acetylation before dehydrogenation/ ... 

Dihydrophenanthrene (3) is dehydrogenated without added metal salt. In this case the intermediate I I complex (b) is suggested. 

Dehydrogenation of the dihydrophenanthrene intermediate formed by the cyclization of (Z)stllbene presents a general route to substituted phenanthrenes [81]. Yields can be improved by the addition of hydrogen abstractors such iodine or tetracyanoethene... 

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