Spiro pyran

Benzoquinone, having a low triplet energy (Et= 50 kcal/mole),<104) reacts with dienes to yield spiro-pyrans, presumably through the following allylic radical intermediate 51 ... 

The 3,4,5-tri-O-benzyl D-fructopyran derivative should lead to the spiro-pyran forms. Selective deprotection of the 4,5-O-isopropylidene, followed of perbenzylation and condensation with thiocyanic acid gave the expected spiro-pyran OZT in reasonable yield (53%), but standard acetylation was not very efficient for the separation of the two epimers (Scheme 26). 

The 3,4-di-O-benzylated D-fructopyran derivative (Scheme 26) would limit the possibilities for spiro-furan and -pyran derivatives. Application of the usual two-step process afforded with a 60% overall yield a mixture of OZTs in which the spiro-furan forms predominated (2 1 ratio) over the spiro-pyran forms. After column chromatography separation, acetylation allowed isolation of each epimer either a-furan (37%) and (3-furan (35%) forms as well as a-pyran (41%) and (3-pyran (27%) forms. 

If the triplet energy of the carbonyl compound is below that of the diene, triplet-triplet transfer will become inefficient and photocycloaddition may occur. For example, the triplet energy of 1,4-benzo-quinone is about 50 kcal mole - V41 and photocycloaddition to dienes can indeed occur.42 The products are spiro-pyrans 13 (not vinyl oxetanes), which may arise via the allylic radical intermediate 12. 

Photochromism of Spiropyrans. The reversible photochromism of spiro-pyrans has been exhaustively studied by Fischer and his colleagues58 and shown to be a reversible transformation between the spiropyran form A (colorless) and the merocyanine form B (colored). 

P. Allegrini, N. Noderi, V. Malatesta, Mid L. Crisci, Spiro-pyranic compounds endowed with photochromic characteristics, Eur. Pat. Appl. 0,625,518,A1 (1994). 

Figure 6. Hyperflne spectral parameters (coupling constants in gauss = 10 4 tesla) for the spiro-pyranic aminyloxide 73 in benzene solution.

M. Matura, Effect of polystyrene matrix on decolouration reaction of coloured spiro-pyran, Polymer 19 (1978) 348. 

Scheme 28 Spiro pyran-quinoline synthesis by cationic BINAP-Pd(II)...

Schematic representation of the concerted phenomenon coupled with the charge transfer phase transition in [Fe"Fe" (dto)3] and the photoisomerization of spiro-pyran in (SP)[Fe"Fe" (dto)3]. White and black circles represent the Fe" and Fe " sites, respectively. (Reproduced from Ref. 34 with permission of Elsevier.)...

Stitzel S, Byrne R, Diamond D (2006) LED switching of spiro-pyran-doped polymer films. J Mater Sci 41 5841-5844... 

Hobley J, Malatesta V, Miilini R, Montanari L, Neil Parker WO (1999) Proton exchange and isomerisation reactions of photochromic and reverse photochromic spiro-pyrans and their merocyanine forms. Phys Chem Chem Phys 1 3259-3267... 

One photoactive material is based on compounds called spiro-pyrans , i, the name reflecting their twisted shape As you can see, in... 

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