Dihydropyrans Subject

In addition to its other properties, interest in the potential use of the vasodilative properties of prostaglandin El, alprostadil ( ), has led to several conceptually different syntheses.For this purpose, the classic Corey process has to be modified by reversing the order of addition of the side chains to allow for convenient removal of the unwanted double bond in the upper side chain. For example, Corey lactone is protected with dihydropyran (acid catalysis), reduced to the lactol with diisobutyaluminum hydride, and then subjected to the usual Wittig reaction to give intermediate This is... 

A more useful way of reducing esters to ethers is a two-step procedure applied to the reduction of lactones to cyclic ethers. First the lactone is treated with diisobutylaluminum hydride in toluene at —78°, and the product - a lactol - is subjected to the action of triethylsilane and boron trifluoride etherate at —20° to —70°. y-Phenyl-y-butyrolactone was thus transformed to 2-phenyltetrahydrofuran in 75% yield, and 5-lactone of 3-methyl-5-phenyl-5-hydroxy-2-pentenoic acid to 4-methyl-2-phenyl-2,3-dihydropyran in 72% yield [1034]. 

Under Simmons-Smith conditions, bis-dihydropyran 566 was smoothly cyclo-propanated but 570 was also subject to transannular reactions under certain con-... 

Oxidative fluorination. "y,8-Difluoro-ct, 3-unsaturated esters are produced at a Pt anode when dienoic esters are subjected to electrolysis in MeCN containing 3HF-EtjN. A chemical process employing 3HF-Et,N and NXS converts dihydropyran into 2-fluoro-... 

The already described alcohol (23), prepared from Wieland-Miescher ketone (1) was made to react with dihydropyran to obtain (170) which on subjection to hydroboration-oxidation yielded the alcohol (171). Its mefliyl derivative (172) was converted on oxidation to the ketone (173). The formyl derivative of (173) was subjected to annelation reaction following the procedure of Howell and Taylor [74]. The resulting adduct... 

For this study, the commercially available triacetyl D-glucal (17) (Table II) was the first substrate subjected to the cycloaddition conditions. Unfortunately, under the conditions used for furanoid glycals, the cycloaddition did not take place. However, when the reaction was carried out at 90 C a 3/2 diastereomeric mixture of adducts 27 was obtained in low yield. Since dihydropyran was found to be a good substrate in the cycloaddition reaction under irradiation, this suggested that the low reactivity of triacetyl D-glucal was due to the acetoxy groups on the ring. 

The transition-metal-catalyzed olefin metathesis is a still recent, but already well-established, development, which has evolved toward a highly useful synthetic tool [27]. The ring-closing variant of this reaction ]ring-closing metathesis (RCM)] has proved to be very suitable for the preparation of carbo- and heterocycles of any ring size, except for those that are strained ]28, 29]. In the case of 5,6-dihydropyran-2-ones, RCM of homoaUyl acrylates has very often been used for the direct creation of this heterocyclic system, most particularly in the past decade (Figure 2.4). Allyl esters of fi,y-unsaturated acids may also be subjected to RCM with the formation of P,y-unsaturated 8-lactones, which can be easily isomerized by acid or base catalysis to their a,fi-unsaturated counterparts ]6, 30-39]. 

Figure 2.5 illustrates the first five of these particular reaction types. As shown, these methods require precursors of different structural types and aHbrd dififerent products. Thus, intramolecular HWE olefinations may yield 5,6-dihydropyran-2-ones directly, as do methods (3-5). The Baeyer-Vilhger reaction, however, provides, in most cases, tetrahydropyran-2-ones, which must be subsequently subjected to dehydrogenation. 

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