Decatrienones

Rh2(OAc)4-catalyzed decomposition of m-methoxy-substituted diazoketones 244 furnishes 6-methoxy-2-tetralones rather than the expected bicyclo[5.3.0]decatrienones. This demonstrates that the direction of ring opening in the norcaradienone intermediate 245 may well be influenced by the nature and position of a substituent. 

Decomposition of l-diazo-4-arylbutan-2-ones offers a direct entry to bicyclo[5.3.0]decatrienones and the approach has been extensively used by Scott and coworkers to synthesize substituted azulenes.137 Respectable yields were obtained with copper catalysis,137 but a more recent study24 showed that rho-dium(ll) acetate was much more effective, generating bicyclo[5.3.0]decatrienones (154) under mild conditions in excess of 90% yield (Scheme 34). The cycloheptatrienes (154) were acid labile and on treatment with TFA rearranged cleanly to 2-tetralones (155), presumably via norcaradiene intermediates (156). Substituents on the aromatic ring exerted considerable effect on the course of the reaction. With m-methoxy-substituted systems the 2-tetralone was directly formed. Thus, it appeared that rearrangement of (156) to (154) was kinetically favored, but under acidic conditions or with appropriate functionality, equilibration to the 2-tetralone (155) occurred. 

Intramolecular Buchner reaction. Rh(OAc), is more efficient than CuCl as the catalyst for cyclization of the a-diazo ketone 1, derived from 3-phenylpropionic acid, to bicyclo 5.3.0]decatrienone (2). This product isomerizes in the presence of triethylamine to the more stable trienone 4. A useful isomerization to 3-tetralone (3) occurs in the presence of trifluoroacetic acid. 

Decatrienones and Other Trienes with Unsaturated Connecting Chains Intervention ef Boat-like 539... 

The increased stereoselectivity of decatrien-3-ones relative to the n(matrien-3-ones may also be the consequence of cooperativity between twist asynchronicity and endo stabilization that both favor the c -fused product. The decatrien-3-one transitirm state is considerably less strained than the nonatrien-3-one transition state, and the uncatalyzed decatrienone cyclizations occur at or near ambient temperature. Consequently, one expects greater endo stabilization, and hence also greater cis stereoselectivity, in the decatrienone intramolecular Diels-Alder reactions. 

Conformational effects also play an important role in determining IMDA diastereoselectivity. For example, trienes like (49) and (50) with two sp hybridized atoms within the connecting chain cyclize preferentially to ci s-fused rather than the trans-fuscd products that would be expected based tm the presence of the C(8)-methyl substituent (Figure 15). The benzo fusion forces the bridging chain to adopt boat-like conformations in the transition states, and the 1,3-diaxial interactions that the C(8)-methyl ordinarily experiences in the cif-fused, chair-like transition state are thus relieved (refer to Figure 12). The preferential formation of the cif-fused diastereomer may then be the consequence of a favored skew butene rotamer at C(6)—C(7) in (51) compared with a less favorable gauche butene rotamer at this position in the trans-fused transition state. This effect is discussed in more detail in Section 4.4.3.2 concerning relative diastereoselection in the IMDA reactions of substituted decatrienones. 

Relative diastereoselection is an issue that is confrtHited most frequently in the IMDA reactions of trienes with one or more stereocenters on the chain connecting the diene and dienophile. As a general rule, trienes with saturated amnecting chains cyclize via transition states with chair-like conformations, while boat-like transition states are frequently encountered with the 1,7,9-decatrienones as well as with... 

Zhu, J., Klunder, A. J. H., Zwanenburg, B. Synthesis of 6-functionalized tricyclodecadienones using Barton s radical decarboxylation reaction. Generation of tricyclo[5.2.1.0 ]decatrienone, a norbornene annulated cyclopentadienone. Tetrahedron 1995, 51, 5099-5116. 

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