Endocyclic position

Apart from PET-reductive cyclization, chemical reduction has also been applied to the total synthesis of natural products such as capnellenediol 186 [184]. Naphthalene sodium is shown to be a suitable oxidant for generating ketyl radical anions which cyclize efficiently in a 5-exo-dig mode. In contrast, electroreductive cyclization of 184 does not lead to 185, but exclusively to the thermodynamically preferred 5-exo isomer with a remaining double bond in the endocyclic position [185] (Scheme 35). The steroid precursor 4.5-secocholes-tan-5-one 187, in which the lOa-side chain is varied, has been cyclized under the same conditions [186-188] (Scheme 36). Reduction with naphthalene sodium or sodium in ether exclusively produces the A B-cis steroid 188 with an exo double... 

A variety of alkanes can be dehydrogenated with catalysts such as 1. Whilst COA dehydrogenation usually stops at the COE stage, cyclohexane dehydrogenation affords benzene in good yields [7]. Substituted cyclohexanes are dehydrogenated at both exocycHc and endocyclic positions for example, ethylcyclohexane dehydrogenation produces styrene as the major product (Equation 12.1). 

Ring contraction also resulted on irradiation (185 nm) of cyclopentenes, to form methylenecy-clobutanes in low yield. Thus, photolysis of cyclopentene gave methylenecyclobutane (36) in 27% and bicyclo[2.1.0]pentane in 28% yield,110 via an intermediate carbene shown by rearrangement of [3,4-2H2]cyclopentene. Labeling in the product was found almost exclusively in endocyclic positions,111 Analogously, 1 -methyl-1 -vinylcyclobutane (37) was obtained from 1,2-dimethylcyclopentene in 20% yield.112... 

The six-membered OMCTS B can be dilithiated at two endocyclic positions, yielding an isomer Bl", 20.5 kcal/mol above the most stable species, and at one... 

Especially interesting are the cases not observed by Gilli which extend the line xNi = 2 tni c2 up to Ixn I = 80 ° and tni c2 = 40 °. In these cases the endocyclic position of N1 in the rigid systems enforces on it a very high pyramidality. 

Whereas the carbonyl group of the accepting enone etc. must necessarily be exocyclic, the C-C double bond may be in an exo- or endocyclic position. In the following paragraphs additions to exocyclic C-C double bonds are discussed before endocyclic ones. 

This method depends on the isomerization of the exocyclic double bond at position 3 of a compound of type 52 to an endocyclic position. 

Pyranoid sugar conjugates having an exocyclic, enolacetal double bond were postulated as intermediates in the biosynthesis of 6-deoxy-L-hexoses (for example in strains of Escherichia coli)Pyranoid derivative 229, bearing an exocyclic, enolacetal linkage, is formed from 228, and is transformed by way of 229 into the isomeric compound (230) having the enolacetal double bond in the endocyclic position. 

A combined study on 2-acetyltetralone using laser flash photolysis and time resolved EPR shows formation of triplet excited state. From these results, it is concluded that the molecule is in the enol form, with its double bond in endocyclic position. ... 

The final class of monocyclic [3]dendralenes, the 1,3-analogs 22, can undergo DA reactions at either their endocyclic position (in a diene-transmissive sense)... 

Both compounds 138 and 139 are important in relation to the formation of cyclopentadiene-type structures. Introduction of a double bond in an endocyclic enolized 2-acyl-1,3-cyclopentanedione system immediately produces rearranged structures. When the double bond is in the endocyclic position, the enolization of the... 

To avoid synthetic problems, we prepared aldehyde 47, a substrate in which isomerization of the C-C double bond to the endocyclic position is less likely due to the extended conjugation of the alkene moiety. Compound 47 was obtained as a pure substance and also presents a substitution pattern that, according to the above postulates, should favor the ODPM rearrangement. However, irradiation of 47, using m-methoxyacetophenone as sensitizer, led to formation of the diene 48 (38%), as a result of photodecar-bonylation (Scheme 9). No ODPM product was formed in this process.Irradiation of the corresponding methyl ketone 49, under the same conditions as used for 47, afforded the product of 1,3-acyl migration 50 in 24% yield (Scheme 9). Again, no ODPM product was formed in this instance. The formation of 48 and 50 is reminiscent of Norrish Type 1 processes. However, in these cases, homolytic bond fission does not occur in the carbonyl nit excited state, as is the case in normal Norrish Type 1 reactions. ... 

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