Cyclization photoreductive

Intensive studies concerning the photoreductive cyclization of distinct ketones and aldehydes are made by Cossy et al. [170], They describe how bicyclic tertiary cycloalkanols 173 and 174 can be prepared from, s-un-saturated ketones 172 in good yields, initiated by photoinduced electron transfer from triethylamine in acetonitrile or by photoionization in pure hexamethyl-phosphoric triamide (HMPA) [171, 172], The reaction is stereo-, chemo- and... 

A photoreductive strategy for a short total synthesis of the sesquiterpene ( )isoafricanol was developed by Cossy et al. Thereby, the hydroazulenol skeleton was generated by photoreductive cyclization of an unsaturated ketone employing triethylamine as the electron donor (Scheme 40) [61],... 

Belotti, D., Cossy, J., Pete, J.P., and Portella, C. (1986) Synthesis of bicyclic cyclopentanols by photoreductive cyclization of 8,e-unsaturated ketones. Journal of Organic Chemistry, 51, 4196-4200. 

The synthesis of bicyclic cyclopentanols (46) via photoreductive cyclization of 5,8-unsaturated ketones (45) has been realized by Belotti et al. using hexamethylphosporic triamide (HMPA) or triethylamine (TEA) as electron donor [46]. The photocyclization proceeded remarkably efficient when HMPA was used as donor and solvent (Sch. 24). Furthermore, only one stereoisomer was obtained carrying methyl and hydroxy groups in trans-configuration. In contrast, the yields for cyclization dropped significantly when TEA in a polar solvent such as acetonitrile was used. As an example, the yield for 46 (n= 1) decreased from 81% in HMPA to 50% in TEA/ MeCN. 

Scheme 74) [330], The use of photoreductive cyclization for the total synthesis of actinidine and isooxyskytanthine has been reported [331],... 

A study of the photoreduction of the oxoamides (103) in acetonitrile/triethylamine has shown that the oxoamides (103 a,b) afford the alcohols (104) in high yield. However, the amides (103 c,d) and (105) are also reactive and afford the cyclized compounds (106 - 108) respectively. The failure of the oxoamides (103 a,b) to undergo cyclization is presumed to be due to intramolecular hydrogen bonding. The ease with which this reductive cyclization occurs with amides or unsaturated ketones has been exploited in a new synthesis of hirsutene (109). Thus irradiation (254 nm) of the ketone (110) in acetonitrile/triethylamine affords the alcohol (111, 20X) and the desired cyclic compound (112, S8X). This latter material is readily converted to hirsutene. The ketone... 

A polymer containing pe,ndant benzophenone groups (XV) is found to photocrosslink. The mechanism is inter.molecular coupling of radicals formed via photoreduction in the triplet state. In solution, crosslinking is concentration dependent as intramolecular cyclization competes (21). 

Facile synthesis of isooxyskytanthine (300), a monoterpene alkaloid, was performed by photoreductive intramolecular cyclization of the 5-oxocyclopentanecarboxamide... 

In one recent synthesis, 91 was prepared using a photoreductive cyclization of a N,N-unsaturated dialkyl-2-oxocyclopentanecarboxamide [274]. In another synthesis of 91, a mixed copper, zinc organometallic reagent was added to a 2,5-disubstituted V-acylpyridinium salt to... 

The addition of radicals, generated by photoelectron transfer, to an unsaturated functional group has received the most attention. The fate of the intermediate radical does not depend strongly on the method used to produce it. An interesting example of a photoreductive reaction of co-unsaturated ketones in the presence of hexamethylphosphoric triamide (HMPA, neat) or triethylamine (Et3N) in acetonitrile has been reported [33]. The thus formed <5, e-unsaturated ketyl radicals exhibit the same behavior as m-unsaturated radicals [34] and they cyclize to produce cy-clopentanols and cyclohexanols. A typical example is provided in Scheme 18. 

The linear triquinane skeleton is easily accessible from methyl 8-oxotricyclo-[5.4.0.0] undecan-l-carboxylate 67 [64]. Photoreduction of 67 affords ring-expanded intermediate 67" which cyclizes to the linearly fused triquinane 68 (Scheme 29, eq 1). An extension of this reaction to the heterocyclic angular tricyclic compound 70 has also been achieved starting from keto lactone 69 (Scheme 29, eq 2) [75]. 

Studies of similar reactions under a variety of conditions had been previously reported. Thus, electroreductive, photoreductive, as well as metal-induced ketyl-olefm cyclizations had all been explored prior to our investigations. Many of these cyclization reactions using simple unsaturated ketones took place with reasonably high diastereoselectivity at two stereocenters (Eq. 3). This feature of the transformation was ascribed to favorable secondary orbital interactions between (he developing methylene radical center and the alkyl group of the ketyl, and/or to electrostatic interactions in the transition state leading to product. ... 

The photo-induced cyclizations of the selenol esters (20) and (21) to give (22) and (23), respectively, have been described. Photolysis of benzyl phenyl selenide (24a) or of the phenyl ribosyl derivative (24b) yield the corresponding diselenides (25). The photoreduction of acetophenone by H2Se has been investigated. ... 

An interesting feature shown in Table 11 is that, with PVK copolymers (structure VII), (t> co corresponds almost exactly to c )oh. suggesting that photoreduction (or cyclization) is the major route for loss of carbonyl in these polymers. However, with PIPK copolymers, <(> co i four times as great as < )oH- We suggest that in this case, because of the greater stability of the tertiary radical formed, the major loss of carbonyl in these polymers (structure Vni) is by the Norrish type-I reaction. Because of mobility restrictions for the type-II process in the solid phase, the quantum yield < >] appears to be significantly higher than in solution. 

The reaction proceeds by electron donation from excited HMPA to the ground state of the ketone, or by reduction of the excited ketone by ground state El,N. Good yields and stereoselectivity are observed under these mild reaction conditions. If HMPA gives better yields in the cyclization of unsaturated ketones, the two solvent systems, however, are equally effective in the case of acetylenic ketones. However, for photoreduction with EtjN or HMPA, EtjN is preferable since this compound is less hazardous, more volatile, and allows a simpler reaction work-up (only requiring evaporation of the solvent). From a... 

This photoreductive cyclization is of great synthetic utility as illustrated below in the synthesis of actinidine and isooxyskytanthine, two rare monoterpenic alkaloids which... 

The synthesis of the skeleton of these two monoterpenic alkaloids and the control of the relative stereochemistry at C4, CS, and C6 was aehieved using a Wolff rearrangement followed by a photoreductive cyclization of unsaturated Af-alkyl-2-oxo-cyclopentane carboxamides of type D (Scheme 40). 

A Wolff rearrangement applied to the diazodiketone 96 in the presence of diallylamine gave a 1.6/1 mixture of two re-gioisomeric amides 97a and 97b (Scheme 41). The major product 97a, which is the precursor of actinidine, was isolated in 56% yield. Similarly, photolysis of 96 in the presence of N-methyl propargylamine gave almost quantitatively a mixture 99a and 99b (1.5/1). The photoreductive cyclization of 97a carried out in the presence of Et N led to a 1.7/1 mixture of 98a and 98b which was transformed to actinidine. Irradiation of 99a under the same conditions led to a single product 100, which could be converted to isooxyskytanthine (Scheme 41). 

Iridoids represent a class of highly oxygenated monoterpenes characterized by a c/s-fiised cyclopentapyran ring system. In principle, synthesis of such systems could be achieved by a photoreductive cyclization of S,e-unsaturated p-ketoesters. Unfortunately, unsaturated p-ketoester 101 did not lead to the ex-... 

The skeieton of linear triquinane is easily accessible from the methyl 8-oxotricyclo I 5.4.0.0l undecan-l-carboxylate 150." Photoreduction of 150 implied the formation of a ring-expanded intermediate K which cyclized to produce the linearly fused triquinane 151 (Scheme 58). An extension of this reaction to heterocyclic angular tricyclic compounds has also been achieved (Scheme 59). This photofragmentation method has been ex-... 

The photoreductive cyclization of 2,3-unsaturated-C glycosyl carbonyl compounds is depicted in Scheme 4P... 

---