Five Ring Oxygen Systems

Hosokawa, H. Ohkata, and I. Moritani, Bull. Chem. Soc. Japan., 1975, 48, 1533. 

Hosokawa, M. Hirata, S. I. Murahashi, and A. Sonoda, Tetrahedron Letters, 1976, 1821. M. Bertrand, J. P. Dulcere, G. Gil, J. Grimaldi, and P. Sylvestre-Panthet, Tetrahedron Letters, 1976, 1507. 

Pterocarpans (61) are usually made by acid-catalysed cyclization of 2 -hydroxy-flavanols, but the latter are not usually accessible with the additional hydroxy, methoxy, or methylenedioxy groups in the required position for natural products 

3-dioxepane (62) has been resolved via its a-phenylurethane, and isomerized quantitatively by distillation in the presence of a trace of hydrochloric acid to the 

3-dioxolane (63) the latter is a fragment of known absolute configuration corresponding to carbon atoms 5 to 12 of the maytansinoid ring skeleton.  

---

Besides the bond-pair cheletropic disconnection of oxiranes and aziridines to an alkene and "atomic oxygen" (from a carboxylic peracid) or a nitrene, respectively, and the hetero-Diels-Alder cycloreversion, of special interest are the 1,3-dipolar cycloeliminations of five-membered rings [-(34-2)] leading to 1,3-dipoles and an unsaturated acceptor or dipolarophile. So large is the number of different five-membered heterocyclic systems resulting from 1,3-dipolar... 

Furan and thiophene rings are electron-rich, five-membered aromatic systems. The donation of a pair of electrons by the heteroatom into the 7t-system is essential to the aromaticity of these compounds. Due to the highly electronegative oxygen heteroatom, the aromaticity of hiran is comparable to pyrrole. The aromaticity of benzofuran is comparable to indole for similar reasons. On the other hand, the aromaticity of thiophene resembles benzene due to the decreased electronegativity of the sulfur heteroatom. For similar reasons, the aromaticity of benzothiophene is comparable to naphthalene. ... 

The ligand effect seems to depend on the substrates. Treatment of the prostaglandin precursor 73 with Pd(Ph3P)4 produces only the 0-allylated product 74. The use of dppe effects a [1,3] rearrangement to produce the cyclopen ta-none 75(55]. Usually a five-membered ring, rather than seven-membered, is predominantly formed. The exceptionally exclusive formation of seven-membered ring compound 77 from 76 is explained by the inductive effect of an oxygen adjacent to the allyl system in the intermediate complex[56]. 

Boyd G. V. Five-Membered Ring Systems With Oxygen and Nitrogen Atoms... 

---