Ring expansion/contraction

Ring Expansion/Contraction via Sigrnatropic Rearrangements - Cope Rearrangement... 

More recently, the same principle was applied by the same authors to cyclic alkanes for catalytic ring expansion, contraction and metathesis-polymerization (Scheme 13.24) [44]. By using the tandem dehydrogenation-olefin metathesis system shown in Scheme 13.23, it was possible to achieve a metathesis-cyclooligomerization of COA and cyclodecane (CDA). This afforded cycloalkanes with different carbon numbers, predominantly multiples of the substrate carbon number the major products were dimers, with successively smaller proportions of higher cyclo-oligomers and polymers. 

Kricheldorf HR, Lee S-R, Schittenhehn N (1998) Macrocyclic polymerization of (thio) lactones - stepwise ring expansion contraction. Macromol Chem Phys 199 273-282... 

The oxetane /-amides 222 undergo a ring expansion-contraction sequence in the presence of a Lewis acid to azetidine derivatives 223 (Equation 60) <2000JOC2253>. The overall reaction sequence has been described as double isomerization . The four-membered oxetane ring first enlarged to a [2.2.2]-dioxazabicycle, which in turn rearranged to the final azetidine derivatives. 

Ring contractions of cyclobutanediol monotosylates provide a stereocontrolled, preparative entry to highly strained cyclopropyl systems. Treatment of 7-(4-methylphenylsulfonyl)bicyclo-[3.2.0]heptan-l-ol with potassium /err-butoxide results in simultaneous ring expansion-contraction, yielding pww-fused bicyclo[4.1.0]heptan-2-one (9)62. 

Similarly, 10-(4-mcthylphenylsulfonyl)bicyclo[6.2.0]decanol undergoes ring expansion-contraction upon exposure to boron trifluoride or alumina yielding /erm-fused bieyclo[7.1.0]decan-2-one (10)63. 

Introduction of a linear alkene or polyalkene to these systems was shown to rapidly affect the selectivity of the ring-expansion/contraction process through the formation of linear chains as side products. 

Ring Expansions, Contractions, and Rearrangements. l,3,4,5-Tetrahydro-henzo[f>]azepin-2-ones or 1,2,3,4-tetrahydro-henzo[d]azepin-2-ones are accessible from simple tribromomethyl-suhstituted A benzyl quinolines, or isoquinoline precursors via a AgNOs-promoted ring enlargement. The reaction was used for the synthesis of the antianginal zatebradine (eq 10). 

Scheme 1 Reactions constituting ring-chain equilibria in ROP (simplified scheme). In brackets, a variant with hyper-bonded structures, for example, with oxonium cations or catalyst coordinated (activated) compounds, is shown. Z denotes an initiator originated end group (X-) or a linear fragment of polymer (X(M)r). Reversible ring-expansion/contraction reactions were added to this general scheme, although they are usually possible and/or important only in some coordination ROP polymerizations.

Thomas and Kendrick observed the establishment of the ring-chain equilibria in systems of linear poly(dimethylsilox-ane) at high temperatures without catalysts and interpreted their results as the effea of a siloxane-bond interchange reaction involving a four-center transition state. This reaction is in fact a ring-insertion/exclusion reaction (Scheme 3) (or ring-expansion/contraction reaction if only tydic compoimds are involved, cf. Scheme 4) ... 

If only cyclic compounds are present, the reactions of ring expansion/contraction lead to ring-ring equilibria. However, the authors did not give any evidence that the high-molar-mass product had cyclic stmcture. Therefore, one cannot exclude that the high-molar-mass product observed by... 

Scheme 4 The ring-expansion/contraction reactions proposed by Bannister and Semlyen for poly(dimethylsiloxane) systems.

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