Diene syntheses intermediates prepared

Diels-Alder cycloadditions have greater synthetic flexibility when suitable substituents on the diene are available for elaboration to functionality which is otherwise difficult to build into the structure. To this end, several schemes to novel 1,3-dienes have been described this year. Hagemann s ester (79) is a useful intermediate in synthesis of terpenoids. The ester is now conveniently available by regioselective Diels-Alder reaction of 1-methyl-l,3-bis(trimethylsiloxy)buta-1,3-diene (80) with ethyl acrylate. The yield is 61% overall after 5 days heating in xylene at 170—180 C, followed by hydrolysis of the moisture-sensitive adduct (81). The diene (80) is readily prepared from acetylacetone and trimethylsilyl chloride in the presence of triethylamine-zinc chloride in an ether-benzene mixture. The related diene (82), similarly prepared from 2-formylbutan-3-one, is used to give the useful cyclic dienophile (83). These sequences are outlined in Scheme 18. ... 

Cope itself. For example, Evans and coworkers exploited the thermal rearrangement of diene 83 for preparation of the cw-decalin system 84, an intermediate in the synthesis of ( )-luciduline, a Lycopodium alkaloid. 

Substituted cyclobutenes, which are useful intermediates for the synthesis of 2-substituted 1,3-dienes, can be prepared regioselectively in high yield from 1,1-bis-selenocyclobutanes and one of a variety of electrophiles e.g. Scheme 30)/" The key step, regioselective introduction of a double bond within the ring, is accomplished with potassium hydride or potassium t-butoxide in DMSO, or with t-butyl hydroperoxide and alumina in THF, depending on the nature of the substituent. 

Grignard reactions have been used in the preparation of l-(dialkyl-phosphinyl)- ,3-dienes, e.g. (11), which are intermediates in the synthesis of antiflame polymers. Double-resonance n.m.r. techniques have been applied to the study of the reactions between thiophosphoryl halides and Grignard reagents. For example, the reaction between methylmag-nesium iodide and t-butyldibromophosphine sulphide gave the mixed... 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

Generally, trialkylboranes are useful intermediates in the field of organic synthesis with versatile reactivity. The polymers prepared by polyaddition between diene monomers and thexylborane are polymer homologues of trialkylboranes, which can be converted to poly(alcohol)s, poly(ketone)s, and other polymers having some functional groups (scheme 4).8-12... 

In acyclic systems the 1,4-relative stereoselectivity was controlled by the stereochemistry of the diene. Thus, oxidation of (E,E)- and (E,Z)-2,4-hexadienes to their corresponding diacetates affords dl (>88% dl) and mesa (>95% me so) 2,5-diacetoxy-3-hexene, respectively. A mechanism involving a t vans-accto xy pal I adation of the conjugated diene to give an intermediate (rr-allyljpalladium complex, followed by either a cis or trans attack by acetate on the allyl group, has been suggested. The cis attack is explained by a cis migration from a (cr-allyl)palladium intermediate. The diacetoxylation reaction was applied to the preparation of a key intermediate for the synthesis of d/-shikimic acid, 3,... 

The successful application of this method was illustrated with a series of model dienes, and with the formal synthesis of leukotriene A4 methyl ester, a complex polyene monoepoxide, from the intermediate epoxyundecadienoate 17 prepared by selective epoxidation of trienyl ester 16 (equation 17). 

The versatile cyclohexa-1,4-diene 32a has served as an intermediate for synthesis of (—)-eburnamonine 81 and the Aspidosperma alkaloid (—)-aspidospermidine 84 (Scheme 18).3s Butyrolactone carboxylic acids 78 and 82 were prepared from 32 by modification of the methodology outlined in Scheme 7. The key Pictet-Spengler-type cyclization of 79 under conditions of kinetic control gave an 18 1 mixture of 80 and its C(3) (3-epimer in 93% yield. Subsequent hydroboration... 

The key intermediate in the synthesis of the derivatives of 19-F3-androstane is the trifluoro analogue of the Wieland-Miescher ketone. Its preparation involves a Diels-Alder reaction between a trifluoromethyi ketone and a siloxy diene. Another original step is the regioselective reduction of a diketone only the ketone function in P of CF3 (probably activated by this substituent) is reduced (Figure 4.6). " Then, a succession of classical reactions leads to derivatives of androstane from the trifluoro analogue of the Wieland-Miescher ketone (Figure 4.7). ... 

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