Acyclic Diene Stereochemistry

It is possible to prepare 1-acetoxy-4-chloro-2-alkenes from conjugated dienes with high selectivity. In the presence of stoichiometric amounts of LiOAc and LiCl, l-acetoxy-4-chloro-2-hutene (358) is obtained from butadiene[307], and cw-l-acetoxy-4-chloro-2-cyclohexene (360) is obtained from 1.3-cyclohexa-diene with 99% selectivity[308]. Neither the 1.4-dichloride nor 1.4-diacetate is formed. Good stereocontrol is also observed with acyclic diene.s[309]. The chloride and acetoxy groups have different reactivities. The Pd-catalyzed selective displacement of the chloride in 358 with diethylamine gives 359 without attacking allylic acetate, and the chloride in 360 is displaced with malonate with retention of the stereochemistry to give 361, while the uncatalyzed reaction affords the inversion product 362. 

The stereochemistry of both chlorination and bromination of several cyclic and acyclic dienes has been determined. The results show that bromination is often stereo-specifically anti for the 1,2-addition process, whereas syn addition is preferred for 1,4-addition. Comparable results for chlorination show much less stereospeciftcity. It appears that chlorination proceeds primarily through ion-pair intermediates, whereas in bromina-hon a stereospecific anfi-l,2-addition may compete with a process involving a carbocation mtermediate. The latter can presumably give syn or anti product. 

One of the many photoreactions of conjugated dienes is the formation of cyclobutenes,99 and this has been reported both in cyclic and acyclic dienes. The stereochemistry of such photocyclizations has been discussed in terms of orbital symmetry by Woodward and Hoffmann.100... 

While the process works for a great number of conjugated dienes, a few, such as 1,3-cyclopentadiene and those acyclic dienes that have an oxygen substituenl in an allylic position, did not give a chloroacetoxylation product.23 Control of the 1,4-relative stereochemistry and preparation of compounds analogous to the title compounds also work for acyclic dienes,23 5 This process was used to obtain remote stereocontrol in acyclic systems and applied to the synthesis of a pheromone.5... 

Nucleophilic ring opening of the lactone ring in bicycloadducts of type 5 leads directly to tetrasubstituted cyclohexenes in which the relative stereochemistry of all four contiguous stereocenters is established. Thus, pyrones provide attractive synthetic equivalents to acyclic dienes of type 6 which may be difficult to prepare as pure geometrical isomers and which in many cases do not lead via Diels-Alder cycloaddition to the desired stereochemical relationships. The application of [4+2] cycloaddition reactions of 2-pyrones to synthesizing functionalized cyclohexenes was the partial subject of a 1994 review. ... 

Important classes of olefin metathesis reactions include cross metathesis (CM), ring-closing metathesis (RCM), " acyclic diene metathesis polymerization (ADMET), and ringopening olefin metathesis polymerization (ROMP). > This chapter focuses on the last class of metathesis reactions, ROMP, and in particular on the stereochemistry of the resulting macromolecular products. 

Radical Cyclizations. Several examples of radical cycliza-tions have been reported during the selenosulfonation of cyclic or acyclic dienes. When the tether linking the two alkene moieties includes a heteroatom, the procedure affords the corresponding heterocycles. Illustrative examples are shown in eqs I4 i7.3,22d,22b,22f jjj gq quatemization of the nitrogen atom with a Lewis acid affects the yield and stereochemistry of the radical cyclization. Similar cychzations occur when unconjugated enynes or bisallenes are subjected to selenosulfonation. 

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,... 

All of the photochemical cycloaddition reactions of the stilbenes are presumed to occur via excited state ir-ir type complexes (excimers, exciplexes, or excited charge-transfer complexes). Both the ground state and excited state complexes of t-1 are more stable than expected on the basis of redox potentials and singlet energy. Exciplex formation helps overcome the entropic problems associated with a bimolecular cycloaddition process and predetermines the adduct stereochemistry. Formation of an excited state complex is a necessary, but not a sufficient condition for cycloaddition. In fact, increased exciplex stability can result in decreased quantum yields for cycloaddition, due to an increased barrier for covalent bond formation (Fig. 2). The cycloaddition reactions of t-1 proceed with complete retention of stilbene and alkene photochemistry, indicative of either a concerted or short-lived singlet biradical mechanism. The observation of acyclic adduct formation in the reactions of It with nonconjugated dienes supports the biradical mechanism. 

The dimethyl sulfide-sulfur trioxide complex also functions as a thiosulfenylating agent, e.g. converting 1,5-cyclooctadiene into a single adduct, presumably of trans stereochemistry (Scheme 26),37 giving the trans adduct of 2-butyne (equation 6),38 and 1,4-adducts with acyclic 1,3-dienes (Scheme 26).37 With cyclopentadiene both 1,2- and 1,4-addition occurs (Scheme 26).37... 

Total simple diastereoselectivity is observed in the hetero-Diels-Alder cycloaddition of nitro-soarencs to 1,3-dienes leading to the corresponding 3,6-dihydro-2//-l, 2-oxazines. A number of acyclic 1,4-disubstituted dienes have been reacted with nitrosoarenes, but the stereochemistry of the adduct 1 has not been assigned 6 s. 

The stereochemistry of the dialkoxylation arises from two external attacks by the alcohol, one on the r-diene complex and the second on the intermediate 7r-allyl complex. This is in accordance with the other palladium-catalyzed 1,4-syn additions discussed above. Also, the 1,4-dialkoxylation of acyclic 1,3-dienes was stereoselective. For example, the reaction of ( , )-2,4-hexadiene gave the d,l products 38 by a 1,4-syn addition. The double bond was exclusively of E configuration (equation 18),... 

In a similar manner to the fatty acids, monohydric, dihydric and phenolic members invariably exists as a mixture of saturated, monoene, diene and triene constituents and rarely in the pure form as one of these four. In the nomenclature of this series a comprehensive system which takes account of the chain length, double bond position and its stereochemistry has not been widely agreed and accepted. Trivial and systematic names thus both abound and because of their dual functional nature as aromatic and acyclic these compounds have remained relatively unclassified. For example, cardanol is a well-known member consisting of saturated, monoene, diene... 

In Rainier s synthesis of the hemibrevetoxin B ring system,diene 169, which is obviously related to Danishefsky s diene, reacted with O-benzyl 4-hydroxybenzaldehyde, in the presence of zinc chloride, to give 170 in 92% yield. Pyrones and pyrans are particularly useful for the preparation of acyclic precursors to macrocyclic antibiotics. The stereochemistry of the highly oxygenated acyclic fragment can be controlled by exploiting the selectivity inherent in the cyclic pyran (sec. 6.5). 478 xhe disconnection for this reaction is... 

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