Dienes photochemical reactions

A steroid very closely related structurally to cholesterol is its 7 dehydro derivative 7 Dehydrocholesterol is formed by enzymatic oxidation of cholesterol and has a conju gated diene unit m its B ring 7 Dehydrocholesterol is present m the tissues of the skin where it is transformed to vitamin D3 by a sunlight induced photochemical reaction... 

The photochemical reactions of organic compounds attracted great interest in the 1960s. As a result, many useful and fascinating reactions were uncovered, and photochemistry is now an important synthetic tool in organic chemistry. A firm basis for mechanistic description of many photochemical reactions has been developed. Some of the more general types of photochemical reactions will be discussed in this chapter. In Section 13.2, the relationship of photochemical reactions to the principles of orbital symmetry will be considered. In later sections, characteristic photochemical reactions of alkenes, dienes, carbonyl compounds, and aromatic rings will be introduced. 

Photochemical reaction of the ester 114 afforded the alkene 115 and three products derived from 115. A mechanism, involving dimerization of 114 leading to a dithietane intermediate 116, was proposed. Trapping of active sulfur species, generated from 116, with dienes was also described (75CB630). 

Allylic bromination of pregnenolone acetate with dibromodi-methylhydantoin affords the 7-bromo compound (155) of undefined stereochemistry. Dehydrobromination by means of collidine followed by saponification affords the 5,7 endocyclic cis,cis-diene, 156. This compound contains the same chromophore as ergosterol, a steroid used as a vitamin D precursor. The latter displays a complex series of photochemical reactions among the known products is lumisterol, in which the stereochemistry at both C9 and Cio is inverted. Indeed, irradiation of 156 proceeds to give just such a product (158). This reaction can be rationalized by... 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength light.179 Relatively little preparative use has been made of this reaction for simple alkenes. Dienes can be photosensitized using benzophenone, butane-2,3-dione, and acetophenone.180 The photodimerization of derivatives of cinnamic acid was among the earliest photochemical reactions to be studied.181 Good yields of dimers are obtained when irradiation is carried out in the crystalline state. In solution, cis-trans isomerization is the dominant reaction. 

The [2+2]-photocycloaddition of carbonyl groups with olefins (Paterno-Buchi reaction) is one of the oldest known photochemical reactions and has become increasingly important for the synthesis of complex molecules. Existing reviews have summarized the mechanistic considerations and defined the scope and limitations of this photocycloaddition73. Although this reaction likely proceeds via initial excitation of the carbonyl compound and not the excited state of the diene, the many examples of this reaction in natural product synthesis justify inclusion in this chapter. 

The literature of mechanistic aromatic photochemistry has produced a number of examples of [4 + 4]-photocycloadditions. The photodimerization of anthracene and its derivatives is one of the earliest known photochemical reactions of any type97. More recently, naphthalenes98, 2-pyridones" and 2-aminopyridinium salts100 have all been shown to undergo analogous head-to-tail [4 + 4]-photodimerization. Moreover, crossed [4+4]-photocycloaddition products can be obtained in some cases101. Acyclic 1,3-dienes, cyclohexadienes and furan can form [4 + 4]-cycloadducts 211-214 with a variety of aromatic partners (Scheme 48). 

Metallocyclopentenes are frequently formed in photochemical reactions of the Group 14 metal alkyls or catenates in the presence of dimethylbutadiene. This class of compound also has an extensive photochemistry82. For example, photolysis of 51 (R = H or Me) produced the allylic alcohols 52 and 53 and, for R = H, 54. These alcohols could be dehydrated over AI2O3 to give the germole 55 along with other diene compounds. 

In the simple four-electron systems, a route for cis-trans isomerisation of a diene is made available by the photochemical reaction usually being a disrotatory ring closure and the thermal reaction being a conrotatory ring opening ... 

There are a variety of photochemical reactions that non-conjugated dienes can undergo. One of these that is currently of considerable interest is the reactivity brought about by electron-accepting sensitizers such as the cyanoarenes. The photoreactivity of these systems involves the photochemical excitation of the sensitizer to an excited state7. Thereafter, the reactivity is dependent on the ease of oxidation of the alkene or diene. With the transfer of an electron from the diene to the photoexcited sensitizer a radical cation is formed. It is this intermediate that brings about the various processes which occur within the diene systems under investigation. 

In general, the rearrangements of dienes and polyenes can be both thermal and photochemical reactions (the latters are not included in this chapter), and can be catalyzed by acids, bases, metal complexes and enzymes. They can be degenerate processes or occur with the introduction or elimination of functional groups, be accompanied by shifts of multiple bonds or by migrations of atoms or groups and they may lead to cyclizations. 

A photochemical reaction in which the reactant is converted into an isomer. The most common example of such a reaction is photochemical cis-trans isomerization. Another example of photoisomerization is the effect of light on cholesta-3,5-diene. 

Our initial studies focused on the transition metal-catalyzed [4+4] cycloaddition reactions of bis-dienes. These reactions are thermally forbidden, but occur photochemically in some specific, constrained systems. While the transition metal-catalyzed intermole-cular [4+4] cycloaddition of simple dienes is industrially important [7], this process generally does not work well with more complex substituted dienes and had not been explored intramolecularly. In the first studies on the intramolecular metal-catalyzed [4+4] cycloaddition, the reaction was found to proceed with high regio-, stereo-, and facial selectivity. The synthesis of (+)-asteriscanoHde (12) (Scheme 13.4a) [8] is illustrative of the utihty and step economy of this reaction. Recognition of the broader utiHty of adding dienes across rc-systems (not just across other dienes) led to further studies on the use of transition metal catalysts to facilitate otherwise difficult Diels-Alder reactions [9]. For example, the attempted thermal cycloaddition of diene-yne 15 leads only... 

Until recently, the hypothesis that the termination reaction of type II photooxygenation reactions occurs between the substrate and an excited light absorber-oxygen complex seemed to be well established. The typical products obtained from cyclic 1,3-dienes and olefins (see Fig. 1 and Sect. IV) could only be made by photochemical reactions. 

In a [2 + 2] cycloaddition example, 1,2-dichloroethene was reacted with readily available dimethyl 7,8-bis(trifluoromethyl)-9-oxatricyclo[4.2.1.02 5]nona-3,7-diene-3,4-dicarboxylate in a photochemical reaction, from which dimethyl 4,5-dichloro-9,10-bis(trifluoromethyl)-ll-oxa-tetracyclo[6,2.1.02 7.03,6]undec-9-ene-3,6-dicarboxylate (22) was isolated in 72% yield.29... 

Cyclobutane formation via light-induced [2 + 2] cycloaddition is probably one of the best studied photochemical reactions and has been reviewed thoroughly up to 1972 (Houben-Weyl, Vols. 4/5 a and 4/5 b). The most important types of C —C double-bond chromophores undergoing such reactions arc alkenes, 1,3-dienes, styrenes, stilbenes, arenes, hetarenes, cycloalk-2-enones, cyclohexa-2,4(and 2,5)-dienones, 1,4-benzoquinones, and heteroanalogs of these cyclic unsaturated carbonyl compounds. For p notocyciodimerizations see Houben-Weyl, Vol. 4/5 a, p 278 and for mixed [2 + 2] photocycloadditions of these same chromophores to alkenes see Section 1.3.2.3. 

The dienes, specially 1,3-pentadiene (piperylene) and hexadiene, quench the triplets of suitable sensitizers by energy transfer with unit efficiency. Hence, they are used widely in mechanistic studies of photochemical reactions, either to count the triplets or to establish the triplet energy of a sensitizer whose Et is not determinable from spectroscopic data (chemical spectroscopy). 

Such reactions may occur thermally or photochemically, and the differences between the two normally show up in two ways. First, in a thermal reaction the direction of change will be towards the equilibrium position, favouring the more thermodynamically stable compound. whereas in a photochemical reaction the direction of change will be towards a photostationary state that favours the compound with the lower absorption coefficient at the wavelength of irradiation. It is therefore normal for conjugated dienes to be converted efficiently into cyclobutenes using wavelengths that are absorbed bv the diene but not by the cydoalkene 12.12). 

Photochemical electrocycltc ring-closure in a 4-electron system works well for many acyclic dienes (2.17) and related cyclic systems 12.18). The situation with conjugated trienes is more complex, and they can act as 6-electron systems (2.19) leading to cydohexa-1,3-dienes, or as 4-electron systems (2.20) giving cyclobutenes. In addition they can undergo other photochemical reactions such as geometrical isomerization about the central double bond Iwhich must be c/s if a 6-electron electrocydic ring-closure is to take place). 

Photoadditions that arise by initial excitation of the aromatic compound are not common. Benzvalenes are readily attacked by hydroxylic compounds, and so irradiation of benzene in aqueous solutions of acetic acid, for example, results in the formation of a bicydic product (and an isomer derived from it by subsequent photoisomerizationl as a result of addition to the initially formed valence isomer (3.38). A different kind of photoaddition occurs when benzenes react photochemically with amines cyclohexa-T, 4-dienes are the major products (3.39), accompanied by cyclohexa-1.3-dienes, and unlike many of the photochemical reactions of benzene this does not suffer loss of efficiency in scaling-up. 

The molecular geometry of the lepidopterene skeleton remains a feature of unique importance for the observed photochemical reaction. The photolytic An + 27i cycloreversion generates an electronically excited product in which the diene and dienophile moieties are bound to face each other in an arrangement which, subsequent to deactivation to the Franck-Condon ground state, is an ideal one for bond formation. 

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