1- Alkoxy-1,2-dihydrofurans

This methodology is exemplified by two relatively recent syntheses of pyridoxol analogs. For the synthesis of [CD3]-pyridoxal 39b, the requisite deuterated oxazole 37, readily available from deutero-L-alanine, underwent Diels-Alder cycloaddition when heated to 135 C with a 20-fold excess of alkoxy dihydrofuran 38 to provide a 1 1 mixture of the regioisomeric pyridines 39a and 40 in 50-60% yield (Fig. 3.12)." Interestingly, the undesired regioisomer 40 can be quantitatively converted into 39a by treatment with ethanolic HCl. Treatment of 39a with dilute acid then provided hemiacetal 39b in 35-42% overall yield from the deutero-alanine. This route to 39a is clearly superior to the three-step conversion of pyridoxol to [CD3]-pyridoxal, which proceeds in 3% overall yield." ... 

An /V-allylaminium cation radical undergoes an addition-cyclization reaction with vinyl ethers in the presence of t-BuSH to give 3-alkoxy-4-methylpyrrolidines 72 (Scheme 23) in fair to good yields (90TL1675). In addition to EVE, 2,3-dihydrofuran and 3,4-dihydro-(2//)pyran also undergo reactions to give bicycles 73 and 74, respectively, in reasonable yield. 

Alkoxy-2,5-dihydrofurans were oxidatively cleaved to p-alkoxy-substituted a, 3-unsaturated y-ketoaldehydes by using DDQ, as shown below. Presumably, the reaction occurred via the corresponding 3-alkoxyfuran as a transient intermediate <07AGE1634>. However, (5-alkoxybutenolides were obtained from these substrates under Mn(III)-catalyzed ally lie oxidation conditions <07SL1294>. 

Reissig showed that when a variety of 3-alkoxy-2,5-dihydrofurans were subjected to manganese(lll)-catalyzed allylic oxidation, /3-alkoxylbutenolides were obtained as can be seen in Equation (184). The total synthesis of ( )-annularin H was achieved employing this procedure as a pivotal step <2007SL1294>. 

Reissig also oxidatively cleaved the aforementioned 3-alkoxy-2,5-dihydrofurans to lead to the formation of a,/3-unsaturated 7-ketoaldehydes, as can be seen in Equation (185) <2007AGE1634>. 

Heterocyclic cyclopentyl radicals formed in the solvomercuration/reductivc alkylation reaction of dihydrofuran give products with tram- selectivity in a slightly higher ratio than the corresponding carbocyclic analogs34. This is attributed to anomeric effects, which lead to a more pronounced axial orientation of the /J-alkoxy substituent in the tetrahydropyranyl radical. 

Winslow et investigated the use of tetracarbethoxyfuran as a diene, and as expected it does not react with maleic anhydride. However, the tetra alcohol, which can be obtained by the LiAlH4 reduction, reacts easily. From the point of view of preparative utility, alkoxy- and acetoxyfurans are valuable as dienes. They have recently been obtained relatively easily from 2,5-dialkoxy- or 2,5-diacetoxy-dihydrofurans. The normal products with maleic anhydride, maleim-ide, fumaronitrile, and dimethylacetylene dicarboxylate have been described. The stereochemistry of the products was not... 

Dihydrofuro[2,3-b]naphthols, derived from 3,7-dihydroxy-2-naphthoic acids are sources of hetero-fused naphtho[2,l-b]pyrans through reaction with propynols. The oxacyclic substituent is equivalent to an alkoxy group and in the only data provided, the /-fused dihydrofuran derivative 45 exhibits a 9 nm red shift to 481 nm compared with the 8-methoxy analogue 46 <99WOP24438>. 

Synthesis of dihydrofurans substituted in position 2 05EJO4929. Synthesis, properties, and application of 5-alkoxy-2,5-dihydrofuran-2-ones 05CJO239. 

The intramolecular variant of the alkylidene carbene cyclization is achieved by treating functionalized alkynyliodonium salts with a suitable nucleophile. These cyclizations are exemplified by the following works the preparation of various functionalized 2,5-dihydrofurans by treatment of 3-alkoxy-l-alkynyl-(phenyl)iodonium triflates with sodium benzenesulfinate [1002], employment of the alkylidene carbene cyclization in the total syntheses of natural products agelastatin A and agelastatin B [1003] and preparation of the tricyclic core of ( )-halichlorine through the use of an alkynyliodonium salt/alkylidenecarbene/1,5 C—H insertion sequence [1004]. In particular, Wardrop and Fritz have employed the sodium benzenesulfinate-induced cyclization of alkynyliodonium triflate 751 for the preparation of dihydrofuran 752 (Scheme 3.295), which is a key intermediate product in the total synthesis of ( )-magnofargesin [1002]. 

The halogen and the acetoxy group infuse hardness in the ipso carbon, rendering it a better partner for the boron. Although alkoxy [including intracyclic analogs (41) such as 2,3-dihydrofuran and dihydropyran] and amino substi-... 

A cyclopropenation involving selective insertion of alkylidene carbenes into the Ca-Si bond of a-silyl ketones has been achieved by the reaction of TMSC(Li)N2 with a-silyl ketones that are prepared by the indium chloride-catalyzed reaction between TMSCHN2 and aldehydes (eq 28). In the case of a-silyl-a -alkoxy ketones, the Cy-H insertion competitively occurs and a mixture of cyclopropenes (Ca-Si insertion products) and dihydrofurans (Cy-H insertion products) are formed in preference of the former (eq 29). 

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