2//-Pyranes, electrocyclic ring opening

Pyrans and napthopyrans (chromenes) are photochromic compounds that undergo photochemically induced electrocyclic ring opening to give colored ortho-quinone methides.95-98 For example, chromene 153 opens on irradiation to give 154 (Eq. 1.41). 

The same authors extended the [2 + 2 + 2]-cycloaddition methodology to the use of highly electron-deficient tricarbonyl compounds such as ketomalonates (Equation (33)).360 In that particular case, the reaction does not stop at the initial stage of 277-pyrans 164. Instead, a thermally induced electrocyclic ring opening occurred to form the corresponding cyclopentenes 165 as final product. 

A related 1,7-cyclization has been invoked to account for the formation of diazepine 292 from the electrophilic diazoalkane substitution reaction of ethyl diazoacetate and dimethyl diazomethylphosphonate, with the 2,4,6-trimethylpyr-ylium salt 289 (337) (Scheme 8.70). While the expected 4-(diazomethyl)-47/-pyran 290 could be isolated (20-22%), the 2-substituted isomer 291 was not. It was proposed that this latter species underwent 671-electrocyclic ring opening followed... 

The literature on derivatives of 2/f-pyran is relatively sparse, reflecting the overall instability of this system. The heterocycle undergoes facile electrocyclic ring opening to the dienone (6) and generally only 2,2-disubstituted derivatives are stable under normal conditions. However, it has been suggested that it is steric destabilization of the dienone, rather than a specific substitution pattern, which is essential for the existence of the pyran (71JOC1977). 

The 277-pyran system can undergo relatively facile electrocyclic ring opening to form m-2,4-dienones 1, and as a consequence, simple 277-pyrans are commonly present as a mixture of their cyclic and acyclic isomers (Equation 1)... 

Spirocyclobutene systems 3 undergo electrocyclic ring opening in toluene under reflux to form m-2,4-dienones 4, which spontaneously recyclize to afford highly functionalized 2//-pyrans (Scheme 1) <2003T2001>. 

The 2/7-pyran system can undergo relatively facile electrocyclic ring opening to form m-2,4-dienones and several syntheses of 2/7-pyrans are based on the preparation of the acyclic precursors 213 in the hope that the dienone ZH-pyran equilibrium will favor the heterocycle 212. Such dienone precursors can be obtained by Knoevenagel condensation of 1,3-dicarbonyl compounds with, -unsaturated aldehydes (Scheme 118). Simple 1,3-diketones yield the ZH- pyran directly <1988IZV1815> and cyclohexan-l,3-diones afford fused pyrans <1987JOC1972>. 

Spiro-cyclobutene derivatives prepared from the intramolecular Wittig reaction of DMAD with ethyl oxo-(2-oxocycloalkyl)ethanoates yield cycloalka[6]pyrans via a thermal electrocyclic ring-opening - ring-closing sequence (Scheme 1) <03X2001 >. 

D (Scheme 26) [ Paduraru, 2003 671 ]. Oxepinone 832 was reacted with citral (833) to give a mixture of (2//)-pyran 834 and isomeric dienones 835, which cannot undergo electrocyclization. Irradiation of 834 gave [2+2] adduct 836 along with products of electrocyclic ring opening. 

Of the three isomeric naphthopyrans 2-4, the linear isomer 2//-naphtho[2,3-6]pyran 4 displays no significant photochromic response at ambient temperature, a feature which may be rationalised by considering the extensive 7i-system reorganisation which must accompany an electrocyclic ring opening and which would disrupt the aromaticity of both rings of the naphthalene unit. 

Attack of the nucleophile at C-2 and formation of the 2-hydroxy-2/f-pyrans 2 occur first, followed by electrocyclic ring-opening, yielding the products 3 and 4. This ring-opening, which in the case of the unsubstituted pyrylium ion already occurs with H2O, is reversible, i.e. acids reconstitute the pyrylium system from 3 and 4. 

Matrix photolysis of 2i/-pyran-2-one at 8 K leads to electrocyclic ring opening of the bond O/C-2 and formation of the aldoketene 8 which, by addition of CH3OH, can be intercepted as ester 10. Photolysis at higher temperatures or in ether leads to electrocyclization of the 1,3-diene system and formation of the p-lactone 9 which, by addition of CH3OH, yields the enol ether 12. Further photolysis of the P-lactone 9 causes decarboxylation resulting in cyclobutadiene 11 and its products, e.g. the dimer 14 (CHAPMAN) [12] ... 

Organometallic addition takes place at an a-position, or occasionally at C-4 when the a-positions are substituted and C-4 is unsubstituted, or with organocuprates. The initial 2//-pyrans undergo electrocyclic ring opening (and more rapidly than the comparable cyclohexadiene/hexatriene transformation ) affording dienones or dienals. 

A review of the use of electrocyclic reactions in synthesis has been presented" and of asymmetric electrocyclic reactions that result in diastereo- or enantio-selectivity selection as a consequence of the influence of a chiral component, be it substrate or catalyst, on the electrocyclic bond-forming process." Computational studies of 0 electrocyclic ring-opening reactions of 2-pyrone and 6-fluoro-2-pyrone are pseudoper-icyclic 6-fluoro-2-pyranol are a borderline case and 2-pyranol, pyran, and 6-fluoro pyran reactions are pericyclic in character." ... 

When aldehydes were employed in place of CO2 in the cyclization reactions, the formation of dienones was observed. These are presumed to be formed by the electrocyclic ring opening of the proposed pyran intermediate (Figure 13.33) [78]. 

H-Pyrans show the chemical properties ofoxacyclohexadienes for instance, the O/C-2 bond can take part in thermal electrocyclic ring-opening with formation of dienones, for example, 4 5 ... 

Matrix photolysis of 2H-pyran-2-one in CH3OH at 8 K leads to electrocyclic ring opening of the O/C-2 bond and formation of the aldoketene 8, which is intercepted as ester 10 by solvent addition. 

Finally, the spontaneous electrocyclic ring opening of pyran derivatives (obtained by a vinylogous aldol reaction between vinyl malononitriles and aldehydes) into dienamides is to be mentioned in this section [97]. 

Examples of photochemically induced im electrocyclizations in oxygen-containing systems are relatively rare. 2//-Phenanthro[9,10-h]-pyran-4-carboxamides (16), for example, have been obtained in this way by irradiation of dienones (17).17 Similarly, the a-pyran (18) is formed on irradiation of ( )-//-ionone (19)18 a triplet excited state is believed to be involved and the reaction proceeds via the Z-isomer (20). The reverse process involving ring opening is more common and can lead to a wide variety of photochemically derived products. Thus, the products of irradiation of 2,2-dimethylchromene... 

With hydroxide anions (or with water at pH >5) pyrylium salts form pseudobases 16 (2-hydroxy-2//-pyrans) that tautomerize to acyclic 3-penten-l,5-diones 17 via a thermally allowed electrocyclic reaction. Most ring transformation reactions of pyrylium salts involve a similar ring-opening. 

Analogous to the opening of 1,3-cyclohexadiene is the electrocyclic reaction of ring opening of 2H-pyrans XXVI->XXVII ... 

Among the reactions of 4H-pyrans, some ring transformations are synthetically relevant. For instance, the triphenyl-4-benzyl-4H-pyran (1) isomerizes on irradiation to give 2-benzyl-2H-pyran (2), which is converted quantitatively to 1,2,3,5-tetraphenylbenzene (4) on treatment with HCl. This transformation into the benzene system presumably proceeds via an oxa-6jt-electrocyclic opening of 2 to afford the dienone 3, followed by intramolecular acid-catalyzed aldol condensation giving rise to the arene 4 ... 

The photochromic properties of 2/7-chromene derivatives has generated much interest in recent years. Under UV irradiation these molecules can undergo reversible electrocyclic opening of the pyran ring to afford colored ortho-quinone methides <2005T11730, 2005T1681>. 

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