Thermal Opening

Pyrones in this review are thermally quite stable. However, forcing conditions can open the pyrone ring. Thus, the curious isomerization of compounds 344 and 347 has been reported to occur through ketenes 345 and 346 (66M1046). Also, compounds 348 (R = COOEt and benzyl) rearrange to naphthalenes 349 (79CB2756). 

---

Two approaches to convergent steroid syntheses are based on the thermal opening of benzocyclobutenes to the o-quinodimethane derivatives (see p. 80 W. Oppolzer, 1978 A) and their stereoselective intramolecular Diels-Alder cyclizations. T, Kametani (1977 B, 1978) obtained (+ )-estradiol in a six-step synthesis. The final Diels-Alder reaction occurred regio- and stereoselectively in almost quantitative yield, presumably because the exo transition state given below is highly favored over the endo state in which rings A and D would stcrically inter-... 

Thermal opening of the eycloaddition products leads to acyclic sulfones (464,467). 

C—C a bond to be cleft (Fig. 7.34). The LU of the C—C a part will conjugate with the HO of the n part of the ground-state polyene moiety in case of reaction, so that the orbital symmetry relations clearly determines the direction of bond fission. The direction of change is indicated by arrows. In this manner, in the thermal opening, the (4 )-chain will be... 

The approaches of Grieco U5) and Kametani U6) were similar. The benzocyclo-butene (333) was thermally opened and then subsequent intramolecular Diels-Alder reaction afforded (335), which was converted eventually to ( )estrone (332) U5,n6) in fair yields. 

The simplest example of an electrocyclic reaction involving 4n electron system is the thermal opening of cyclobutenes to 1,3 butadienes. The reaction can be done thermally or photochemically and under either conditions, it is stereospecific. 

A o bond of cyclobutene breaks to form the open chain diene. This breaking will be either con -or disrotatory. The conrotatory thermal opening will be as follows ... 

So a thermal opening of cyclobutene takes place in a conrotatory fashion while photochemical inter conversion involves a disrotatory fashion. These generalizations hold good for all systems having 4q 7t electrons where q is any integer like 0, 1, 2 etc. 

When the reaction was run at reflux for 3 hours, besides diester 10%-20% (based on and NMR spectra of crude material) of a thermal-opening product was formed, the structure identical with those independently synthesized by Rh(I) promoted rearrangement of cubane diester 4. 

Few examples of carbonyl ylide additions to Cjq are knovm. A stable tetrahydrofurane adduct 272 (Scheme 4.47) has been prepared based on the generation of the ylide by thermal opening of tetracyanooxiran (TCNE oxide) 270 [316]. 

According to orbital symmetry rules, thermal opening of the nm -aziridine must take place in a conrotatory manner, leading to ds-ylide, and photochemical opening via a disrotatory one gives a Jrafts-azomethine ylide. 

The thermal reaction of 6-amino-4-oxopyrano[3,4-d][l,2,3]thiadiazoles (32) leads to 6-hydroxy-4-oxo-[l,2,3]thiadiazolo[4,5-c]pyridines (33) and 2-cyano-2-(l,2,3-thiadia-zol-5-yl)acetamide (34).58 Formation of (33), a Dimroth-type rearrangement, proceeds by thermal opening of the pyrane ring, followed by the simultaneous rotational isomerization of the ketene intermediate and its recyclization on to the amido group to form the pyridin-2-one cycle. 

What compound will result from the thermal opening of bicyclo[3.2.0]hept-6-en-2-one ... 

In contrast to dioxocins, the thermal opening of diaza and oxoaza benzene tautomers 95 (X = NR, Y = NR or O, R = H) to the corresponding l,4-dihydro-l,4-diazocines or l,4-dihydro-l-oxa-4-azaazocines 94 appears to be irreversible (see also Section 2.5.2.2) <1979AGE964, 1983CB2492>. 

Chelidonine, a representative benzo[c]phenanthridine alkaloid (1,2), was synthesized as the first application of this reaction in natural product synthesis (145). Initial thermal opening of the four-membered ring in 299 led to the formation of a transient -o-quinodimethane (301), which has a dienamide structure in the diene part and is then trapped by the suitably positioned multiple bond in the same molecule. They applied this intramolecular reaction to the acetylenic cyclobutene 300 for the synthesis of ( )-chelidonine (Scheme 110). 

The [2 + 2] cycloaddition of the propynyl tungsten complex (12b) has been found to be stereospecific with the cis and trans isomers of eAyl prc nyl ether." ITie reaction with c/s-propenyl ethyl ether gave rise to the cyclobutenyl complex (18), in addition to a minor amount of the ring-opened dienyl carbene complex (19). The stereochemistry about the double bonds in the dienyl complex (19) revealed that it was derived from a thermal electrocyclic conrotatory ring opening of the c/s-cyclobutenyl carbene complex (18) and thus the stereochemistry is completely retained in this cycloadditirxi. In support of this analysis it was found that cyclobutenyl complex (18) could be thermally opened to the dienyl complex by heating in an inert solvent at 50 C. In the cycloaddition of the propynyl complex (12) with trans-... 

Fig. 18.2. Thermal opening of 1,2-dimethylcyclobutene. The two hydrogens and two methyls are forced into conrotatory motion so that the resulting p orbitals have the symmetry of the HOMO of the diene.

Thermal openings of cyclopropyl ketones involve homo[l,5]-hydrogen shift, which can serve as a prerequisite for ring enlargement (equation 57). 2,3-Disubstituted cyclopen-tanones have also been made by using this rearrangement... 

Reduction of the photo-adduct (446) derived from (+)-isopiperitone and cyclobutene-1-carboxylic acid with NaCNBH3 gives the lactone (447). Thermolysis of this compound affords the 6a-epimer of isoaristolactone (448) and the elemanolide (449).213 A novel approach to the synthesis of germacranes involves the thermal opening of a bridgehead cyclobutene which, in turn, is derived by an oxy-Cope rearrangement (Scheme 57).214... 

By these simple rules Woodward and Hoffmann predicted a disrota-tory course for the opening of the cyclopropyl cation in its ground state to the corresponding allyl cation, while the thermal opening of cyclopropyl radical and anion to allyl radical and anion is conrotatory. A glance at Fig. 2 clearly shows the reason. Reverse predictions can be made for photochemically induced reactions. 

---