Diene-transmissive Diels—Alder

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

Reacting 126 with dienophiles 283 produces Diels-Alder adducts 284, which, as cydobutenes, can thermally be ring opened to the conjugated dienes 285, ready for a repetition of the cycloaddition step to provide the 2 1 adducts 286. Hence the whole sequence represents a diene-transmissive Diels-Alder protocol [118]. 

Tsuge and co-workers - building on work by Bailey and Blomquist [5] - have already made extensive use of the preparative potential of these so-called diene transmissive Diels-Alder additions (DTDA additions) [8]. Clearly, the dienophile does not have to be identical in the two stages of the reaction, which increases the preparative potential of these double cycloadditions considerably. The adducts of type 22 can be further processed in various ways, for example by dehydrogenation to give 1,2,6,7-tetrasubstituted naphthalene derivatives as will be discussed below. Consecutive reactions of this type with their excellent atom economy are of considerable interest particularly in view of the current efforts to increase the efficiency of organic transformations. 

In a study of the diene-transmissive Diels-Alder reactions <90BCJ284>, cycloaddition of a substituted 1,4-dihydrothiadiazine and maleic anhydride or A-(p-methoxyphenyl)maleimide gave substituted bicyclo[4.4.0]dec-10-enes (107) in good yield (Equation (59)) <90BCJ284>. The starting thiadiazine is itself a Diels-Alder adduct of an a,(l a, / -unsaturated thioketone and dead. 

The cross-conjugated thione (20 X = S), prepared from the corresponding ketone (20 X = O) with phosphorus pentasulfide, participates in a diene-transmissive Diels-Alder reaction with diethyl azodicarboxylate <90BCJ284>. The 3,4-dihydro-2//-l,2,3-thiadiazine (21) so formed is isolable (see Section 6.13.7.2.2) and on prolonged heating with diethyl azodicarboxylate undergoes further cycloaddition to yield the pyridazino[3,4-e]-l,2,3-thiadiazine derivative (23) in 74% yield (Scheme... 

Synthesis of the quassinoid system has been approached by a sequence of Diels-Alder reactions in which successive cycloa tions are made feasible by the previous pmcyclic reaction, the so-called diene-transmissive Diels-Alder strategy illustrated below (94JOC5596). 

Kanemasa S, Sakoh H, Wada E, Tsuge O Diene-transmissive Diels-Alder... 

The DTHDA reaction is a special case of the DTDA (diene-transmissive Diels-Alder) reaction in which one or more heteroatoms are contained within a cross-conjugated triene/polyene (heterodendralene) it-system, a dienophile it-system, or both. The DTHDA protocol is an efficient and attractive method for the stereoselective synthesis of a variety of heterocyclic systems. 

Diene-Transmissive Diels-Alder Reactions l41S... 

Furthermore, azo dienophiles have been employed in diene-transmissive hetero Diels-Alder reactions of cross-conjugated trienes which allow the straightforward construction of polycyclic compounds [297]. Theoretical interest has been directed to the hetero Diels-Alder reaction of diethyl azo di-carboxylate with 1,3-cyclohexadiene whose concerted course was demonstrated by means of a high pressure study [298]. 

Such syntheses take advantage of transannular intramolecular hetero Diels-Alder reactions [426], another impressive example is the diene transmissive hetero Diels-Alder reaction of the thioketone 5-14. The diene 5-15 formed by this cycloaddition underwent a second Diels-Alder reaction with N-phenyl-maleimide to yield the fused polycycle 5-16 as single diastereomer (Fig. 5-5) [427]. 

Dielectric constant A value that serves as an index of the ability of a substance to resist the transmission of an electrostatic force from one charged body to another the lower the value, the greater the resistance. Diels-Alder reation A reaction involving the addition of an ethylenic double bond to a conjugated diene. 

A diene-transmissive hetero-Diels-Alder reaction of cross-conjugated azatrienes provides a new, efficient, synthesis of ring-fused nitrogen heterocycles such as pyrimido-[5,4-c]pyridazin-6-ones 31 (Scheme 5) <97CC1013>. 

The Diene-Transmissive Hetero-Diels-Alder Reaction... 

Compounds with certain [3]heterodendralene structures including more complex substructures and substituents such as carbonyls, imines, and ylides are very popular, and hence many reports on their synthesis/generation, application and/or characterization, and so on, were found. However, only a few examples of their diene-transmissive hetero-Diels-Alder (DTHDA) reactions have been reported so far. For instance, although divinyl ketones ([3]-3-oxadendralenes, Scheme 2.1) are well recognized, for example, for their Nazarov cychzation [1] (see Chapter 3), only two reports are known on DTHDA reactions of p,p -diphenyl- and di-2-thienyl-substituted divinyl ketones vide infra) [8]. 

In 2014, Sherbum and coworkers disclosed a short synthesis of (-)-psendopterosins (G-J) aglycone 75 (Scheme 8.10) [30]. To bnild the carbocyclic scaffold, they delineated an unconventional strategy, which relies upon the nse of a diene-transmissive triple Diels-Alder reaction. The enantio-enriched 1,1-divinylallene 78 was prepared in three steps from the chiral propargyl alcohol 77 (chiral pool). This unusual diene was then subjected to a Diels-Alder reaction to provide the cycloadduct 79 in 61% yield and in good diastereomeric ratio. A second [4h-2] cycloaddition of 79 with acrolein using high-pressure conditions took place to afford the expected cycloadduct 80 in 15% yield. Although a... 

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