Conrotatory closure

In contrast to pentadienyl anions, heptatrienyl anions have been found to cyclize readily to cycloheptadienyl anions. The transformation of heptatrienyl anion to cyclo-heptadienyl anion proceeds with a half-life of 13 min at — 13°C. The Woodward-Hoffinann rules predict that this would be a conrotatory closure. ... 

The mechanism of the classic ketene-imine reaction to form /J-lactams [17,18] is thought to involve perpendicular attack of the imine nitrogen on the ketene carbonyl carbon from the side of the sterically smaller of the two groups, followed by conrotatory closure of the zwitterionic intermediate (Eq. 6). This... 

The alternate approach of Dewar and Zimmerman can be illustrated by an examination of the 1,3,5-hexatriene system.<81,92> The disrotatory closure has no sign discontinuity (Hiickel system) and has 4n + 2 (where n = 1) ir electrons, so that the transition state for the thermal reaction is aromatic and the reaction is thermally allowed. For the conrotatory closure there is one sign discontinuity (Mobius system) and there are 4u + 2 (n = 1) ir electrons, so that the transition state for the thermal reaction is antiaromatic and forbidden but the transition state for the photochemical reaction is aromatic or allowed (see Chapter 8 and Table 9.8). If we reexamine the butadiene... 

Figure 11.7 Symmetry at an intermediate stage of the conrotatory closure. Only the C2 axis remains as a symmetry element.

Figure 11.11 Correlation diagram for the forbidden conrotatory closure of hexatriene.

With two more electrons in the conjugated system, making eight in all, the octatetraenes 4.51 and 4.54 show conrotatory closure giving the cyclo-octatrienes 4.52 and 4.55, However, the reaction can only just be stopped at this stage, for the products undergo a second electrocyclic reaction giving the bicyclic dienes 4.53 and 4.56 as a result of the allowed disrotatory reaction of the all-m hexatriene units. 

These results are compatible with stereochemical predictions derived through orbital symmetry theory, assuming a one-step n2s - -n2a addition. But secondary deuterium kinetic isotope effects on the allene plus allene thermal (2+2) cycloaddition seem to require a two-step mechanism with formation of an intermediate 44>, and as Moore and coworkers fully realized 83> stereoselective formation and reactions of 2,2 -biallylene intermediates will equally well account for the product ratios. In their rationale, two allenes approach and distort through simultaneous conrotatory twistings to give the perpendicular 2,2 -biallylene intermediate, which closes to form products in a disrotatory fashion. The experimentally observed stereochemical selectivity is equally compatible with a reversed order of rotatory motions disrotatory joining of two allenic reactants followed by conrotatory closure to create the 1,2-dimethylenecyclobutane products 83>. 

For the 1,2-dimethylenecyclobutane rearrangement, Gajewski and Shih 53> have demonstrated preferred conrotatory ring opening and closing. For the allene-allene cycloaddition, then, the sequence would be disrotatory motion as two allenes approach to form the perpendicular biallylene intermediate, followed by conrotatory closure of that species. 

This type of cyclization has recendy been used in the caibocyclic field to synthesize the tetracyclic glycol (234), an advanced intermediate in the synthesis of the antimicrobial and amoebicidal agent ikaru-gamycin. Thus photocyclization of a 2 1 mixture of trienes (235) and (236) led to a 4 1 mixture of the two possible conrotatory closure products (237) and (238), which likely reflect the conformational bias in the ground state of the (20-triene (23fo and 236b, respectively). ... 

Finally, there has been speculation and recent experimental support for the involvement of a Nazarov-type cyclization in the biosynthesis of c/ s-jasmonic acid ° and marine-derived prostanoids. Radiolabel tracer studies have demonstrated Ae intermediacy of 8-HPETE (104) in the biosynthesis of prostanoid intermediate preclavulone A (lO ). This remarkable conversion was proposed to proceed by formation of allene oxide (105) followed by isomerization to (107) via the 2-oxidocyclopentadienyl cation (106 Scheme 41). To demonstrate the chemical feasibility of this proposal, Corey reported the transformation of epoxysilane (108) to, inter alia, the cyclopentenone (111 Scheme 42). The reaction is presumed to involve formation of the allene oxide (109) followed by isomerization to the 2-oxi-dopentadienylic cation (110). Conrotatory closure of (110) is expected to produce the cis isomer of (111) as observed. 

The formation of the C(l)-C(4) a bond via the participation of two 7T electrons of the HOMO requires that the 2p,r orbitals in atoms C(l) and C(4) undergo rotation in the same sense conrotatory mode) so that positive (in-phase) overlap occurs (bonding effect). If they rotate in opposite directions (disrotatory mode) then there will be negative overlap and an anti-bonding effect. Accordingly, the conrotatory closure of (a) in a thermally controlled reaction leads to the cis isomer of dimethylcyclobutene (b) and not to (c). 

Z)-Diarylethenes, such as stilbene, also undergo a 671-electrocyclization reaction upon irradiation from the singlet n,n state, but not upon triplet sensitization.487 529,599 622 The photochemical orbital symmetry-allowed conrotatory closure leads to the trans-dihydro derivative 66 (Scheme 6.23), which can subsequently be oxidized by oxygen or iodine to the corresponding arene (phenanthrene) in nearly quantitative yield.561... 

For the cyclobutene-butadiene TS, the conrotatory closure results in a Mobius system, whereas a disrotatory TS gives a Htickel system. The same rules of aromaticity apply as for ground state molecules. A Htickel system is aromatic when it has 4 - -2 electrons. A Mobius system is aromatic when it has 4n electrons. In the case of the cyclobutene-butadiene interconversion, which involves four electrons, it is the conrotatory Mobius TS that is the favored aromatic transition state. 

Two classes of photocyclization reactions of indoles are well exemplified in the literature. One is based on a 6-ti electrocyclic closure of trienes to cyclohexadienes where the indole C2-C3 bond forms part of the triene. The other involves photolysis of m-haloalkylindoles whereupon the halogen-substituted carbon becomes attached to an indole ring position. The first example of the former reaction was reported by Szmuszkovicz [75], who found that UV light irradiation of 2,3-bis-(para-methoxyphenyl)indole in the presence of iodine gave the carbazole 71 (Scheme 37). By analogy with the photocyclization reaction of other 1,2-diarylethenes [76], this reaction presumably proceeds via conrotatory closure of the indole singlet excited... 

Finally, tricyclo[4.1.0.0 ]hept-3-ene gives almost exclusively bicyclo[3.2.0]-hepta-2,6-diene upon heating at 150°C with a half-life of 1.5 h (Scheme 8.17). " It was suggested that this rearrangement involves the symmetry-allowed opening of the bicyclobutane to a 1,3,5-cycloheptatriene with one trans double bond followed by conrotatory closure to the bicycloheptadiene. 

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