FUSION TO FOUR-MEMBERED RINGS

The first simple thiirane fused to a saturated four-membered ring is (13), prepared in 34% yield by irradiation of the acyclic sulfide (12) (69JOC896). The parent compound is not known. 

In contrast to (14), several examples of Dewar thiophenes are known. The first example was (17), prepared by irradiation of (16) (72CJC2721, 75CPB2773). The degenerate Sulfur walk to (17 ) occurs at a rate of 55 s at 157 C (77JA629), appreciably faster than thermal 

Fused-ring Oxiranes, Oxirenes, Thiiranes and Thiirenes 

Peroxy acid oxidation of (17) gave sulfoxide (18) whose F NMR spectrum showed equivalent CF3 groups even at —95 °C (76JA4325). Tlie rate ratio for the sulfur walk in (18/17) is an astounding 10 ° at 25 C theoretical reasons for the difference have been discussed (80JA286i). 

Both (17) (74TL2841) and (18) (76JA4325) undergo cycloadditions at the double bond without disruption of the thiirane ring (80JOC2962, 80JA6633). This type of reaction has been used to convert (17) to the Dewar pyrrole (20) via the tricyclic thiirane (19) (77JA7350). 

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Unlike for oxirane fusion to three-membered rings, oxiranes fused to four-membered rings (5-oxabicyclo[2.1.0]pen-tanes, cyclobutene oxides) are well known. 

Chemical shifts for the furans (113a) and (113b) characterized by fusion with the small four-membered rings seem to reveal a substantial upheld shift for the latter that could well indicate a degree of antiaromaticity in the peripheral 107r-electron system (73CC214, 72JA8950). 

XXXVI) (9) leads to products in which a four-membered ring is created by the fusion of two carbon to carbon double bonds. 

The rate of enolization is increased by heating or by base catalysis. The cis ring fusion in the product is more stable than the trans because there are not enough atoms in the six-membered ring to span trans-1,2 positions in the four-membered ring without excessive strain. 

The protons in the 1,2-thiazetidine T-oxide ring 22 appear as a pair of doublets as expected for fit-coupling in a four-membered ring, but significantly they are not coupled to the vicinal bridgehead protons of the norbornene component thereby confirming the fuco-fusion <1997SL167>. 

In derivatives of Dewar benzene, the bond length of the bridging C-C bond is found to lie in the range 156-159 pm. The elongation of bond length is mainly due to the strain caused by the fusion of two four-membered rings. 

Fig. 4 (a), Two-dimensional [NTIytSOUCHeOLf, XIII, formed by the sinusoidal chains which, in turn, consist of hexameric units from ref. 66, (b) (0-T) two-dimensional cadmium sulfate of [Cdj(S04)3(H20)2]2 in XIV, redrawn from ref. 35. Note the fusion of the four-membered ring ladders to form the layered structures. 

Four-membered Rings. These show the expected high frequency shift of about 83 cm" [150]. Such systems, and the effects of ring fusion, have been much studied in relation to penicillin [19—24]. 

The RHF calculation on the thymine dimer in the previous section showed that the ring fusion at the C5 and C6 atoms of two thymine bases created the four-member cyclobutane puckered ring. The same feature is observed with the CAS(3e+4o) level. The puckering leads to axial or equatorial directions for the substituent atoms on the cyclobutane ring the substituent atoms on the two thymine bases differ in their directionality. Especially noteworthy is the direction of the H6A atom. In the neutral T<>T, the H6A atom is equatorial relative to the four-member ring and axial relative to the 7C conjugation system... 

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