Pyran moiety

In this synthesis, a key step is the intramolecular hydroxymercuration of 135 which results in the formation of the pyran moiety of 136 with good selectivity (85% yield, 86% de). 

Much interest has been devoted to the photochromic behaviour of simple chromenes, especially because of practical applications of spiropyrans, particularly indolinospirans.5 Kolc and Becker30 have been able to demonstrate the o-quinoneallide structure of the colored form 4, by producing it in THF at -75° and trapping by reduction with LiAlHj. It is concluded31,32 that the same intermediates occur when spiropyrans are irradiated, because only the pyran moiety has an... 

We have created efficient syntheses of remarkably potent and selective bifunctional tryptase inhibitors, which are also competitive and reversible, containing pyran moieties and hetero and non-hetero aryl diynes as scaffolds. Several modifications at the core templates and the linker moieties are well tolerated without significant loss of inhibition activity. In contrast with previous results published recently [32], it was also apparent from the aryl diyne inhibitors that the distance between the two terminal amino groups can be considerably less than 30 bonds in highly potent target compounds (e.g. 9 and 29 with 26 bonds each). The in-vitro potencies of the compounds were between 1 im for 26 and 1.3 nM for 15 with high selectivity against other serine proteases (trypsin, thrombin, and factor Xa, respectively) in... 

Labdanes (core C6 C6 linked to variously reduced furan or pyran moieties) include the Lamiaceae (Labiatae) diterpenes forskolin (C6 C6 pyran) (that activates adenylyl cyclase, the enzyme catalysing cyclic AMP formation from ATP) and premarrubiin (C40L G6 C6 furan-furan) that converts to the bitter non-opiate antinociceptive marrubiin (G4L C6 C6 -(CH2)2-furan). 

Spiroketals can also be obtained by cyclization of 4,5-dihydroisoxazoles containing the 3,4-di-hydro-2//-pyran moiety. Thus, treatment of dihydroisoxazole 22 with iodine in dichlorometha-ne results in excellent internal asymmetric induction, whereas no relative asymmetric induction is observed. The corresponding spiroketal 23 is obtained in 57% overall yield as a 50 50 diastereomeric mixture of epimers at C-238. 

The total synthesis of the marine toxin polycavemoside A was achieved by J.D. White and co-workers. In order to couple the central pyran moiety in a Nozaki-Hiyama-Kishi reaction, the aldehyde side chain had to be first homologated to the corresponding terminal alkyne and subsequently transformed into a vinyl bromide. The aldehyde substrate was treated under the Ohira-Bestmann protocol, and the desired alkyne product was obtained in high yield. 

For spirobipyrans consisting of two identical pyran moieties, we assume that the experimental rate constant ke for the enantiomerization is equal to k, the rate constant for the transformation of the spiro ground state to an intermediate, e.g., a merocyanine. The reason for this assumption is the fact that the usual correction50,51 for an intermediate and an additional correction for the two equal possibilities ofC(sp3)-0 bond cleavage in a spirobibenzopyran cancel. 

The 3,4,5-trimethoxyphenyl group is also an excellent replacement for the pyran moiety of FK506, as reported by workers at Vertex Pharmaceuticals.189 Compound 7 is a very potent FKBP inhibitor, and the branched ester semaphore compound 8 is equipotent with FK506 as an FKBP 12 inhibitor and ligand. Replacement of the pipecolic acid ring... 

In 1997, Chida et al. presented a convergent synthesis of (-)-pironetin starting from L-quebrachitol 131 and L-malic acid. The acyclic portion of (-)-pironetin possessing four contiguous chiral centers were constructed stereoselectively from L-quebrachitol and the 2-pyrane moiety was prepared from L-malic acid. In this synthesis, the nucleophilic addition of a dithiane to epoxide is a strategic key step. 

At this point, 1417 was saponified and coupled with the fully elaborated pyran fragment 1418 to afford 1419 (Scheme 1.363). The second oxazole ring was then installed by sequential Dess-Martin oxidation of 1419 followed by cyclization with (CeHslaP, BrCl2CCCl2Br in the presence of 2,6-di-fert-butyl-4-methyl-pyridine. These conditions were critical given the acid-sensitivity of the pyran moiety. 

The plan was to elaborate the pyran moiety throu the aldehyde while the skipped triene would be elaborated through the protected hydroxypropyl side chain. 

The successful implementation of this strategy is shown in Scheme 1.370. The optically pure stannane 1441 was transmetalated in situ with 1442, preparing a borane reagent that condensed with 1440. After methylation, 1443 was isolated in excellent yield with 10.5 1 diastereoselection. At this point, 1443 was elaborated to incoprorate the pyran moiety (five steps), and the side chain was converted to an aldehyde (two steps). Overall, this reaction sequence to 1444 was accomplished in 46% yield. The skipped triene fragment was then incorporated via the anion of sidfone 1445. Saponification of the pivalate ester then completed the synthesis of 1191a. 

The flavonoid structure consists of 2 aromatic rings (namely A and B in Fig. la) connected by a pyrane moiety (C). The presence of several phenolic hydroxyl functions within the ring classifies flavonoids in different families, including flavones, flavanols, isoflavones and flavonols, the latter characterized by a 3-hydroxypyran-4-one ring (Fig. lb). 

Nair and coworkers [19] reported a hetero-Diels-Alder reaction of pentafulvene 36 (acting as a 2% component) with tetracyclic enone 70 (the 4jt-heterodiene), to give the polycyclic cage compound 71 containing a pyran moiety (Scheme 7.18). 

Z (Fig. 4a) and —z (Fig. 4b) directions (see Fig. 1). The figures show specifically the C4—09 bond and the insets the entire molecule. As a general rule, an orientation of the external field that favors a decrease of electron density on the pyran moiety, in particular on the C4-09 bond, favors ring opening. This corresponds to the —z direction and —y directions for which the electron density increases on the nitrobenzene part moiety and decreases on the ind-oline part and on the C4-09 bond of the pyran (see Fig. 4b). 

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