Seven- and Eight-membered Rings

Model studies predicted that SnCl., would give the best 

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In the case of cycloheptenone and larger rings, the main initial photoproducts are the -cycloalkenones produced by photoisomerization. In the case of the seven- and eight-membered rings, the double bonds are sufficiently strained that rapid reactions follow. In nonnucleophilic solvents dimerization occurs, whereas in nucleophilic solvents addition occurs. ... 

In the hydrolysis of neutral tricyclic hexacoordinate spirophosphorane 143 with internal N—>P dative bonds, the six-membered phosphorinane ring is retained during the first stage of hydrolysis. Five-, seven-, and eight-membered rings were preferentially hydrolysed (Equation 4) <1998POL3643>. 

Scheme 6.74 Formation of seven- and eight-membered rings by ring-closing metathesis reactions.

A seven-membered ring is formed in the cyclization of 195 (equation 95)105. The homologue 196 affords the fused cyclooctane 197, together with the cis- and trans-decalinones 198 (equation 96)106. Six-, seven- and eight-membered rings are produced in Lewis acid-catalysed reactions of various cyclohexenones with side-chains terminating in allylic trimethylsilyl groups (equations 97 - 99)107. 

As depicted in Eqs. 6.5—6.7, kinetic resolution of a variety of cyclic allylic ethers is effected by asymmetric Zr-catalyzed carbomagnesation. Importantly, besides six-membered ethers, seven- and eight-membered ring systems can readily be resolved by the Zr-catalyzed protocol. 

In this chapter we provide an overview of studies originating from the above work and directed at the design and development of three new metal-catalyzed cycloaddition reactions, namely the [5+2] cycloaddition of vinylcyclopropanes (VCPs) and rc-systems, the [6+2] cycloaddition of vinylcyclobutanones and re-systems, and the three-component, [5+2+1] cycloaddition of VCPs, rc-systems, and CO. These new reactions provide fundamentally new approaches to a range of problems in seven- and eight-membered ring synthesis. 

M. E. Butcher, J. C. Ireson, J. B. Lee, and M. J. Tyler, Seven and eight membered ring sugars and related systems The synthesis of some septanose rings from dioxepans, Tetrahedron, 33 (1977) 1501-1507. 

Steve Martin of UT Austin has reported (J. Org. Chem. 68 8867,2003) a detailed study of the synthesis of bridged azabicyclic structures via ring-closing alkene metathesis. Some examples of his work include the conversion of 12 to 13, efficiently forming six, seven and eight membered rings. He also demonstrated five-membered ring formation with the conversion of 14 to IS, which has the cocaine skeleton. 

Several elements form homocydic rings. Rhombic sulfur, the thermodynamically stable form at room temperature, consists of S rings in the crown conformation. Unstable modifications, S , are known which include n — 6 through n = 36. In fact, sulfur has more allotiopes than any other element.9 Selenium also forms five-, six-, seven-, and eight-membered rings, but they are unstable wilh respect to the chain form. 

Under the same conditions (LDDB in THF, — 78°C) the reductive lithiation step occurred immediately for seven- and eight-membered rings. With a five-membered ring, cleavage of the aryl-sulfur bond was observed and phenyllithium produced. 

Oxepanes and oxocanes.5 These seven- and eight-membered ring ethers can be obtained by cyclization of a-diazo carbonyl compounds induced with Rh2(OAc)4. Examples ... 

The direct a-alkylation of monoketones normally employs reaction of an alkyl halide or sulfonate with the enolate anion produced using a strong base. This method can be satisfactorily used with symmetrical ketones, which are to be dialkylated with a dihalide, and with intramolecular cyclization reactions, where the formation of five- and six-membered rings is often favored over the formation of three-, four-, seven-, and eight-membered rings (M. Mous-seron, 1937 W.S. Johnson, 1963). Regioselective alkylation of dianions according to Hauser s rule (see p. 9f.) is usually also a satisfactory procedure (F.W. Sum, 1979). 

I) Five-membered ring formation, (2) Six-membered ring formation, (3) Seven- and eight-membered ring formation, and finally (4) Tandem cyclization. 

Miginiac et al. [97] synthesized oxepin 233 and oxocin 234 starting from allyl-silane silyl ether 237, which was condensed with various aldehydes in the presence of Et20 BF3 (1.0 equivalent) (Scheme 13.85). The resulting seven- and eight-membered rings were obtained in moderate to excellent yields. 

Several factors are known to affect the ROP of cyclic esters. The main factors are the reaction conditions, i.e., the nature of the initiator, type of solvent and reaction temperature, and also the ring size of the monomer used and the substituents on the monomer ring [105-107] Cyclic esters of four-, seven-, and eight-membered rings polymerize, whereas the five- membered ester does not. In the case of six-membered rings, the polymerizability depends on the substituents [105],... 

Endoperoxidation is the preferred process for cyclic dienes, and simple five, six, seven, and eight membered ring aliphatic dienes lead, by photooxygenation, only to the corresponding endoperoxides which, except for that of cyclopentadiene, are quite stable (Sch. 17) [28]. 

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