1.6.8- Nonatrienes Diels-Alder reactions

A particularly instructive example is the thermolysis of (Z)-l,3,8-nonatriene in which an intramolecular Diels-Alder reaction competes with a sigmatropic [1,5] hydrogen shift (Scheme 24). The use of high pressure here enables a reversal of the selectivity. At 150°C and 1 bar the [1,5] hydrogen shift passing through a monocyclic transition state is preferred. At 7.7 kbar the intramolecular Diels-Alder reaction is preferred due to its bicyclic transition state. 

This model also predicts that selectivity for the tranr-fused cycloadducts in nonatriene (n = 0) or deca-triene (n = 1) cyclizations should increase as size of the coefficients at C 2)IC S + n) are increased relative to those at C(l)/C(9 + n), that is, as the polarization of the dienophile or diene is increased. Tables 1 and 2 summarize results of intramolecular Diels-Alder reactions that provide a test of this propo-gai.24.25 it ig tiiat an electron-releasing Et N group at C(9) of the nonatrioioate system leads to a substantial increase in selectivity for the trans-fased product (compare entries 4-6, Table 1). Increased trans stereoselectivity also occurs with C(9)-alkoxy-substituted nonatrienes. A similar effect... 

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. 

Tab. 2.8. Volume data of the intramolecular Diels-Alder reactions of nonatriene (E)-42 and decatriene (f )-45.

A retro Diels-Alder reaction can account for the vinylcycloheptatriene, but it is also possible that a homo-1,5-hydrogen shift to bicyclo[3.2.2]nonatriene occurs, and this material could be in equilibrium with 7-vinylnorcaradiene (by a 3,3-shift) which can give the more stable 7-vinylcycloheptatriene. The latter process has AG — 35 kcal/mol although the 7-vinylcycloheptatriene has been shown to give 3-vinylcycloheptatriene and bicyclo[3.2.2]nona-2,5,7-triene in a 1 3 ratio in poor yield at 150°C. ... 

Chiral thiopyran derivatives have been obtained in Diels-Alder reactions from monosaccharide O-thioformates and dithiooxalates. Intramolecular Diels-Alder cyclization of 1,6,8-nonatrienes and 1,7,9-decatrienes prepared from pentoses or D-glucose afforded multichiral hexahydroindene and octahydronaphthalene derivatives with very high diastereoselectivity. 

Intramolecular Diels-Alder (IMDA) reactions of 1,6,8-nonatrienes and 1,7,9-decatrienes have been used frequently for the construction of chiral ring systems of complex natural products (18-20), Although saccharide derivatives have been employed many times in intermolecular Diels-Alder reactions (21-28) their IMDA reaction appears not to have been studied so far. The diastereoselectivity of IMDA reactions can be influenced, among other factors, by the size of substituents, and by the chirality of carbons of the connecting chain. Our aim was to study the possibilities of the application of single monosaccharides as tether chains in IMDA reactions. The influence of the presence of three or four chiral... 

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