Pyranopyran

Silyl ethers serve as preeursors of nucleophiles and liberate a nucleophilic alkoxide by desilylation with a chloride anion generated from CCI4 under the reaction conditions described before[124]. Rapid intramolecular stereoselective reaction of an alcohol with a vinyloxirane has been observed in dichloro-methane when an alkoxide is generated by desilylation of the silyl ether 340 with TBAF. The cis- and tru/u-pyranopyran systems 341 and 342 can be prepared selectively from the trans- and c/.y-epoxides 340, respectively. The reaction is applicable to the preparation of 1,2-diol systems[209]. The method is useful for the enantioselective synthesis of the AB ring fragment of gambier-toxin[210]. Similarly, tributyltin alkoxides as nucleophiles are used for the preparation of allyl alkyl ethers[211]. 

Pyrano[4,3-6]pyran-2,5-dione, 7-methyl-aromaticity, 3, 639 Pyranopyrandiones aromaticity, 3, 639 dipole moments, 3, 627 synthesis, 3, 795 tautomerism, 3, 644 Pyranopyranones molecular structure, 3, 624 Pyranopyrans synthesis, 3, 760, 784 Pyranopyrazoles synthesis, 5, 321, 330 Pyranopyrazolones synthesis, 5, 334 Pyranopyridine synthesis, 3, 834 Pyranoses... 

The unsymmetricall allene 189 dimerized to give pyranopyran 190. A hetero-Diels-Alder reaction was followed by the subsequent electrocydic ring closure reaction [8]. 

Dihydropyrano[3,2-r ]chromene-2,5-diones are readily converted into the corresponding dithione with P2SS (Scheme 15) <1997CHE39>. Bromination of the related pyranopyran-2,5-dione using A -bromosuccinimidc (NBS) under thermal conditions gives exclusively the monobrominated product <1998JIC590>. 

Terminal ynamines form 4//-pyrans which are as reactive as the starting ynamine and they react with a further molecule of enone to give pyranopyrans (151). It is also noteworthy that at elevated temperatures pyranopyrans are produced from the reaction of ynamines with methyl vinyl ketone <70CR(C)(271)468). In one instance only is the rearrangement of the pyranopyran to the aminopyran observed. 

The high regiospecificity observed in these reactions corresponds to polarization of the methide as shown (281). This is particularly apparent in the reaction with vinyl ethers ethyl vinyl ether affords the 2-ethoxychroman in quantitative yield and dihydropyran gives the pyranopyran (282 Scheme 74) (70JOC3666). 

The literature relating heterocycles with one or two fully conjugated rings is extensive and is mostly associated with ring synthesis. The chemistry of pyranopyran and pyranopyridine derivatives which are not associated with any fully conjugated rings is also extensive because these heterocycles are present in numerous natural products, and some representative examples are included in this chapter (see Sections 7.15.8, 7.15.10.2, and 7.15.12). 

The chemistry and physical properties of 1,7-naphthyridines (3) <82H(19)363> and of heterocycles (l)-(6) <83AHC(33)147) have been reviewed. The reactions of heterocycles (l)-(6) with nitrogen nucleophiles have also been reviewed <83AHC(33)95>. Reviews covering pyranopyridines (87AKZ104) and pyranopyrans <75MI 715-01) have been published. 

Cyclizations of the type (156) - (151) include the conversion of (a) carboxylic acids (218 X = O, S) into diones (219) (34% yield) under acidic conditions <74MI 715-01) (b) pyridine derivative (220) into the pyrano[3,2-c]pyridine (221) (72% yield) via an intramolecular Grignard reaction <82JCS(P1)93> and (c) a 1 1 mixture of diastereoisomers (222) into a 1 1 mixture of diastereoisomeric pyranopyrans (223) via a HSnBu3-mediated free radical cyclization <93LA629>. The diethyl ethoxymethylenemalonate (EMME) synthesis of 3-ethoxycarbonyl-4-naphthyridone derivatives has been discussed in CHEC-I <84CHEC-i(2)58i>. 

Benzoylacetone reacts with carbon suboxide in the presence of sulfuric acid as a catalyst yielding a mixture of pyranopyran derivatives (383) and (384) <67M243l>. In a related reaction, malonyl... 

Tran -fused pyranopyrans are formed in good yield and with high ee when butenoate esters react with pent-4-en-l-ols in the presence of chiral Cu oxazoline complexes. An initial transesterification is followed by an intramolecular hDA <02TL9397>. 

We were curious to determine whether the chemistry could be extended to more elaborate coupling partners with an eventual goal being to apply it to the total synthesis of natural products containing the pyranopyran framework. Hence, the construction of synthetically useful Q-C-glycosides and their conversion into cyclic ethers became of interest to us. 

Analysis of the x-ray results revealed that all of the stereocenters for compound 134 were as depicted in Scheme 33. The BC-rings were shown to adopt a chair/twist boat conformation to avoid 1,3-diaxial interactions between the methyl group and the benzyloxy unit. A similar conformation was reported for the C-ring in the case of the trans-fvLStd pyranopyran framework [4]. The configuration of the newly formed stereocenter at C in structure 134 is undoubtedly the consequence of a stereoelectronically controlled addition to the cyclic oxonium intermediate, 133. 

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