Pyran tricyclic

An easy, silica gel-promoted 6-endo cyclization of y-epoxy alcohol 32 to pyran 33, followed by acid-catalyzed spiroketalization of the keto diol 34, afforded the common tricyclic spiroketal fragment 35 of lituarines A, B, and C (Scheme 8.8) [20b],... 

Conversion of the A-ring aldehyde 50 to hydroxyallylsilane 57 is accomplished in a three-step sequence involving addition of allylstannane 49 followed by oxidation-reduction for diastereomeric enrichment at Cll. With both hydroxyallylsilane 57 and aldehyde 51 in hand, intermolecular Prins pyran annulation proceeds smoothly to furnish the tricyclic compound 58 in 61 % isolated yield. 

Moreover, unsaturated 3,5-diarylidenepiperidin-4-ones can be substituted for diarylidenecyclohexanones 130 (07BML6459). Analogously, a series of tertralones 132 were employed in the synthesis of tricyclic pyrans 133 (79M115) (Scheme 47). 

The Heck coupling is also efficient in the formation of pyrane derivatives. By starting from the appropriately substituted 1,3,5-tribromobenzene derivative, three fused pyrane rings were constructed in the same reaction. The triple Heck reaction in the presence of palladium acetate and triphenylphosphine led to the formation of the tetracyclic product in good yield accompanied by a minor amount of the tricyclic intermediate(4.12.).13... 

Pentasubstituted tricyclic 4//-thiopyran 48 ( = 2, R = Me) was also prepared by the reduction of the corresponding thiopyrylium salt 227b with LiAlH4.269 The reduction with zinc proceeds via a dimeric 4//-thiopyran intermediate.272 Ion 229 (R = Ph, n = 2) was reduced with zinc in hydrochloric acid to a mixture of 2//-thiopyran 230 (R = Ph, n = 2), 4//-thio-pyran 232, and the corresponding perhydrothiopyran.100... 

Tricyclic 2f/-pyrans 352 and 354 were prepared using the reactions of naphthopyrans 351 and 353 with ethylmagnesium bromide followed by hydrolysis and oxidation.334,335... 

The same approach was used in the synthesis of tricyclic bis-2//-pyran system 525 from thieno-2//-pyran 15.428 The deprotomethylation of 15 was also shown to proceed via intermediates like 517 and 518 and gave various methylated products 526 and 527a-d.430... 

Addition of dichlorocarbene to benzofuran affords the tricyclic species (118) (63JOC577). The furan ring subsequently expands to a pyran, affording 2,3-dichlorochromene (119). This reaction is reminiscent of those shown by pyrrole and indole, which yield pyridine and quinoline derivatives, respectively. However, the product could not be purified and on hydrolytic work-up the dichromenyl ether (120) was isolated. 

Oxazinanes 90 react with 1,3-cyclohexanediones, in acetic acid under reflux, to afford the alkylidene 1,3-diketone intermediate 91. Michael addition of a second equivalent of 1,3-cyclohexanedione then occurs to form the 1,5-dicarbonyl equivalent 92, which upon cyclodehydration furnishes tricyclic 4/7-pyrans (Scheme 30) <1996T14273>. 

Likewise, 1,3-cyclohexanediones react with 3-methyloxazolidines 93 to afford tricyclic 477-pyrans (Equation 48) <1996T14273>. In the presence of malononitrile or ethyl 2-cyanoacetate, 1,3-cyclohexanediones react with oxazinanes 94 to afford bicyclic 2-amino-4/7-pyrans (Equation 49) <1996T14273>. 

Enynals 104 can undergo a palladium-catalyzed cyclization to form the pyrylium intermediate 105, which upon deprotonation affords the triene 106. In the presence of DM AD, a [2+2]-cycloaddition then occurs to form the tricyclic 4//-pyran intermediate 107, which undergoes expansion of the cyclobutene ring to afford the bicyclic 4//-pyrans 108 (Scheme 34) <2004S1409>. 

Chromium- and molybdenum-based carbene complexes condense with ketoalkynes along a CO insertion pathway to afford tricyclic pyran-containing lactone products (29) in variable yield (Equation (9)) (90JA1645, 91JA5459). 

Pterins are redox active in their own right and can adopt one of several oxidation levels, for example, fully oxidized, dihydro, or tetrahydro (Fig. 18). The stereochemical nature of MPT observed in each protein crystallographic study (13) is equivalent to that of the fully reduced, or tetrahydropterin, state. This argues against the existence of dihydropterin states such as 5,6-dihydrop-terin or 7,8-dihydropterin, and one of the various quininoid forms of the dihydropterin, in the crystalline forms of these enzymes that have been characterized by X-ray crystallography. However, it is important to note that, for the tricyclic structure of MPT, the tetrahydropterin state is equivalent to that of a dihydropterin, as manifest in the pyran ring-opened form of the bicyclic... 

Bicyclic and tricyclic cannabinoids, (IV), prepared by Makriyannis (5) were effective as both CBj and CB2 agonists/antagonists and used as an effective and safe treatment option in pain management. Dibenzo[b,d]pyran derivatives, (V), effective as selective CB2 receptor agonists (V) also prepared by Makriyannis (6) were effective in treating... 

The cycloaddition reaction of o-thioquinones with acyclic 1,3-dienes is a finely tuned process that can involve either reagent behaving as the diene component. It has now been established that formation of a 2-spiro-linked thiopyran by a [2-1-4] reaction is kinetically controlled and that the [4-1-2] alternative leads to the thermodynamic product, a 1,4-benzoxathiin (Scheme 31). Cyclic dienes yield only the oxathiin <03T5523>. Incorporation of the thionocarbonyl diene unit into fiiran and pyran rings and reaction with carbohydrate glycols leads to tricyclic oxathianes, desulfurisation of which yields 2-deoxydisaccharides <03T4249>. 

Many rearrangement reactions are initiated by BF3 complexes in the presence of acetic anhydride, including the synthesis of functionalized and optically active pyrans from (+)-(/ ,R)-diethyl tartrate (Eq. 41) [74]. Other examples include the conversion of bicyclic into monocyclic [75] and tricyclic into bicyclic systems [76], the acetolysis of glycosides resulting in the formation of the fully acetylated acyclic derivative [77], and the unusual migration of nitrogen in the dienone-phenol rearrangement of an N-methoxy-/ -lactam [78]. 

The potent neurotoxin (-)-dysiherbaine was synthesized by S. Flatekayama et al. who assembled the central pyran ring via an intramolecular Michael addition of a primary alcohol to an a, 3-unsaturated ester. The sole product of this key cyclization was a tricyclic lactone, which was isolated in good yield. 

Nonclassical cannabinoids consist of bicyclic and tricyclic analogues of A -THC that lack a pyran ring examples include CP55940, CP47497, CP55244 and HU-308 (Fig. 6). They are, therefore, closely related to the classical cannabinoids. 

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