2.2.4.6- Tetramethyl-2//-pyran

Occasionally pyrans result from an aldolization process. Thus 2,2,4,6-tetramethyl-2//-pyran (124) is a byproduct in the production of mesityl oxide and diacetone alcohol from acetone.189,190 Ketol 146 (readily accessible from... 

Tetramethyl-2//-pyran (176) is inert toward ozonolysis in contrast to its double bond isomer 308a.,9° Vicinal dihydroxy-4//-pyran 342 was oxidized with periodic acid to the corresponding dialdehyde.328... 

Tetramethyl-2//-pyran (471b) contained no valence tautomer 470b in benzene.212 Photoinduced E - Z isomerization of 472 led exclusively to 2,2,4,6-tetramethyl-2//-pyran (176), as shown in Eq. (26).397,404... 

Relatively stable 2,2,4,6-tetramethyl-2//-pyran (176) exhibited a high regiospecificity toward methyl propargylate to afford [47t + 27t] adduct 561, which spontaneously decomposed to methyl 2,4-dimethylbenzoate (562) in 73% yield.405 Similarly, labile 3-acetyl-2,6-dimethyl-2//-pyran (564) gave with acetylenic methyl carboxylates benzenoid derivatives 565. On the other... 

Benzo-2//-pyrans and their thia analogs, e.g., heterocycles not treated in this chapter, are photochemically labile and form reversible photochromic systems with open-chain and colored photoisomers. This behavior was observed for 2-benzyl-2,4,6-triphenyl-2//-species 467a,b (Y = H)399,409 and naturally occurring 2//-pyran 92a452a but only at about 77K. At room temperature, these as well as other 2//-pyrans are photochemically stable. Only 2,2,4,6-tetramethyl-2//-pyran (176) was photochemically changed to its exocyclic double bond isomer 308a.404 An irreversible photolysis of 2-azido-2//-thiopyran derivative 212b (R = H) has been discussed in reference 254. 

NMR was used to distinguish 2,2,4,6-tetramethyl-2//-pyran from the isomeric exocyclic methylene compound (32) into which it is converted on treatment with acid (64BSF1492). 

The equilibrium between 2//-pyrans and c/s-dienones is much in favour of the latter species and consequently UV data on these pyrans are scarce. However, steric destabilization of the dienone valence isomer results in the existence of some highly substituted 2//-pyrans either alone or as a mixture with the dienone. The UV spectra of such compounds are similar to those of s-cw-dienes. For example, 2,2,4,6-tetramethyl-2//-pyran absorbs at 221 and 278 nm, and m-/3-ionone, which exists as an equilibrium mixture (Figure 2), at 208 and 253 nm (71JOC1977). 

Photoisomerization of pyran-4-one 301 yields pyran-2-one 302 (Scheme 40). The postulated intermediates involved in this transformation resemble those suggested in the irradiation of pyrylium salts <1984CHEC>. In a related example, irradiation of tetramethyl pyran-4-one 303 gives cyclopentenone 304 via reaction of nucleophilic solvent with the initially formed oxyallyl zwitterion intermediate. 

H-Pyran, 2-alkoxy-4-methyl-2,3-dihydro-conformation, 3, 630 4H-Pyran, 2-amino-IR spectra, 3, 593 synthesis, 3, 758 4H-Pyran, 4-benzylidene-synthesis, 3, 762 4H-Pyran, 2,3-dihydro-halogenation, 3, 723 hydroboration, 3, 723 oxepines from, 3, 725 oxidation, 3, 724 reactions, with acids, 3, 723 with carbenes, 3, 725 4H-Pyran, 5,6-dihydro-synthesis, 2, 91 4H-Pyran, 2,6-diphenyl-hydrogenation, 3, 777 4H-Pyran, 6-ethyl-3-vinyl-2,3-dihydro-reactions, with acids, 3, 723 4H-Pyran, 2-methoxy-synthesis, 3, 762 4H-Pyran, 2,4,4,6-tetramethyl-IR spectra, 3, 593 4H-Pyran, 2,4,6-triphenyl-IR spectra, 3, 593... 

Z)-4,4,5,5-tetramethyl-2-[3- 2-trimethylsilylethoxy)-2-propenyl -1,3,2-dioxaboroUme yield 45% (Z)-2-[3-(tetralnclr -2H-pyran-2-y/oxy)-2-propenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane yield 52%. 

A second powerful route to functionalized allylboron compounds involves the reaction of an a-haloalkylboronatc and a vinyl organometallic reagent3 4-28-29, 50c-92 04. This method is especially useful for the preparation of allylboron compounds not accessible via the allylorganometal-lic route. Notable examples that fall into this category are ( )-4,4,5,5-tctramethyl-2-[4-(tetrahy-dro-2//-pyran-2-yloxy)-2-butenyl]-l,3,2-dioxaborolane (yield 41 %, 93% E) and (E)- or (Z)-2-(l,l-dimethyl-2-butenyl)-4,4,5,5-tetramethyl-1.3,2-dioxaborolane (yield 77-84%. 98% E or 93% Z). 

Relatively few studies of the reactions of allylboron compounds and ketones have appeared. Ketones are less reactive than aldehydes, and as a result these reactions tend to be much slower and often less diastereoselectivc. The reaction of (Z)-4,4,5,5-tetramethyl-2-[3-(tctrahy-dro-2/A-pyran-2-yloxy)-2-propenyl]-1,.3,2-dioxaborolane and ethyl 2-oxopropanoate, for example, was conducted under 6 kbar pressure at 45 C for 80 hours to give a 9 1 mixture of syn-and antz-diastereomers of 1 in 85% yield49. 

T. Liu, C. Iou, S. Wen, and C.H. Chen, 4-(Dicyanomethylene)-2- -butyl-6-( 1,1,7,7-tetramethyl-julolidyl-9enyl)-4//-pyran doped red emitters in organic light-emitting devices, Thin Solid Films, 441 223-227 (2003). 

The regioselectivity of the addition of organometallics to 2,4,6-trisub-stituted pyrylium ions 166 (R2 = R4 = H) is mainly determined by the structure of the entering group R. Thus 2,4,6-trimethylpyrylium perchlorate 177 was found to react with MeMgl, MeLi, or MeNa exclusively to give 2,2,4,6-tetramethyl-2Z/-pyran (176),212,220 237 238,246 whereas i-PrMgX, /-PrLi,... 

Although 2H-pyran itself has not yet been isolated, the H spectra of a number of derivatives which are stabilized by disubstitution at C-2 have been studied. The data for the 2,2,4,6-tetramethyl derivative are shown in Figure 1 and other 2//-pyran H spectra are found in (72BSF707, 81CCC748). 

It is therefore possible to make at least a tentative distinction between 2H- and 4/T-pyrans on the basis of their IR spectra. For instance, 2-hydroxy-2,4,6-triphenyl-2/T-pyran shows peaks at 1610, 1585 and 1575 cm-1 associated with the pyran double bonds and the phenyl groups, whilst 2,4,6-triphenyl-4/7-pyran absorbs at 1640, 1585 and 1575 cm-1 (69TL2195). 2,4,4,6-Tetramethyl-4//-pyran, isomeric with the 2/T-pyran above, exhibits peaks at 1724 and 1667 cm-1, providing further evidence that the highest C=C stretching frequency occurs at higher wavenumbers in 4/7-pyrans than in 2//-pyrans. 

Piperidin 4-Cyan-4-hydroxy-l,2,5-trimethyl- E5, 1417 (On + HCN) Pyran 4-Diazo-3,3,5,5-tetramethyl-tetrahydro- E14b, 1042 (R3P = N-N = CR2)... 

H-Pyran 3,4,6,6-Tetramethyl-5,6-dihydro- VI/4, 92 4H-Pyran 2-Methyl-3-propyl-5.6-dihydro- VI/4, 88 Spiro 2.5]octan 1-Hydroxy-1 -methyl- E17b, 1397 (1,1-Br2 -l-OH/l-CHj)... 

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