Phosphorus compounds Diels-Alder reactions

In the Diels-Alder reaction with inverse electron demand, the overlap of the LUMO of the 1-oxa-l,3-butadiene with the HOMO of the dienophile is dominant. Since the electron-withdrawing group at the oxabutadiene at the 3-position lowers its LUMO dramatically, the cycloaddition as well as the condensation usually take place at room or slightly elevated temperature. There is actually no restriction for the aldehydes. Thus, aromatic, heteroaromatic, saturated aliphatic and unsaturated aliphatic aldehydes may be used. For example, a-oxocarbocylic esters or 1,2-dike-tones for instance have been employed as ketones. Furthermore, 1,3-dicarbonyl compounds cyclic and acyclic substances such as Meldmm s acid, barbituric acid and derivates, coumarins, any type of cycloalkane-1,3-dione, (1-ketoesters, and 1,3-diones as well as their phosphorus, nitrogen and sulfur analogues, can also be ap-... 

Substituted phosphorus heterocycles of varying degrees of unsaturation were easily obtained via Diels-Alder reaction and such adducts may be further aromatized or functionalized to provide phosphorus containing cage compounds < 1996PS(109)425, 1999S644, 1999EJ0363>. 

The ability of carbanions to react with elemental selenium can be advantageously used for the synthesis of selenocarbonyl compounds. For example, sulfur ylides 201 (E = +SMe2) have been reacted with elemental selenium to generate the corresponding selenocarbonyl compounds 202 (Scheme 59).374,375 But Staudinger selenylation also has been applied to the synthesis of selenoketones 202 from phosphorus ylides 201 (E = +PPh3), which have been trapped by dienes in hetero-Diels-Alder reactions.376-383... 

Multiply bonded phosphorus compounds are often reactive as heterodienophiles. However, diis is a diffuse area of research and little systematic study of these Diels-Alder reactions has been done. A complete listing of the various types of phosphorus dienophiles is beyond the scope of diis review. Some of this material is available in previous summaries. Selected representative examples of recent activity in this area is given in equations (109), (110), (111) and (112). ... 

Both cis and rrafw-3-phenoxycinnamic acids cyclize with phosphorus pentoxide to pyran>4-ones but the corresponding phenylthiocinnamic acid isomers behave differently the traits isomer gives a thiopyranone whereas the cis compound produces an indenone. Diels-Alder reactions with heterodienes have been reviewed [24SS, 3067a, 3880]. Indole-3-dithiocarboxylate esters act as dienes in the Diels-Alder reaction with DMAD (cf. Chapter 65, Section II.7). 

Phosphaalkynes also play a prominent role as dienophiles in Diels-Alder reactions hence the X -phosphinines 13 [23] are formed from cyclic 1,3-dienes such as a-pyrone or cyclopen-tadienones by way of extrusion of CO2 or CO, respectively reactions with anthracene provide an access to the phosphabarrelene series [24]. This type of reaction is also of significance for the construction of phosphorus-carbon cage compounds. The same is true for homo-Diels-Alder reactions which, with 2-phosphabicyclo[2.2.2]octa-2,5-diene as reaction partner, lead to the diphosphatetracyclodecenes 14 [25]. Last but not least, ene reactions with phosphaalkynes as enophiles [25] are also valuable for the construction of polycyclic phosphorus-carbon compounds. The reactions of 9 with 2,3-dimethyl-2-butene (- 15) [26, 27] emphasize this behavior. 

Carbon-phosphorus double bonds also participate in the Diels-Alder reaction and are a source of a variety of tetrahydrophosphinine derivatives. The C=P unit has a relatively short history the first compounds were not isolated until 1976. Special characteristics must be present to give stable compounds that resist dimerization or polymerization. Strongly electron-attracting groups or large bulky groups that provide steric retardation of the dimerization or polymerization are required. The former type are the more useful in the Diels-Alder reaction, because steric effects from the bulky substituents can also reduce the reactivity of the C=P unit to additions. An example of a successful cycloaddition is given in Scheme 5.17 (where TMS is MesSi). The literature on Diels-Alder reactions of C=P compounds is the subject of a 2008 review. ... 

Thieno[2,3-6]quinoxalines have been prepared by a method which resembles quite closely the ring closure reaction described in Scheme 111. Heating 3-j8-arylvinyl-2-oxo-l,2-dihy-droquinoxalines (389 equation 42) and phosphorus pentasulfide under reflux in pyridine yields 2-arylthieno[2,3-6]quinoxalines (390). Probably a dihydro compound occurs as an intermediate (80Ci(L)536). Attempts to prepare thieno[3,4-6]quinoxaline (392) by dehydration of sulfoxide (391 Scheme 114) were unsuccessful (77JHC541). Again it was possible to trap (392) as a Diels-Alder adduct (e.g. 393) when (391) was refluxed in acetic anhydride... 

Cycloaddition reactions of two-co-ordinate phosphorus compounds described this year includes the [2 + 2] cycloaddition of an iminoborane to aminoimino-phosphines to give 1,3,2,4-diazaphosphaboretidines (125). In an attempt to prepare hydrazino-bis-phosphaalkenes (126) as new candidates for hetero-Cope rearrangements, the tricyclic compounds (127) were obtained. The result is explained by a Diels-Alder rearrangement of (126), as shown. 

---