Propellane ring system

A related >/4-norcaradiene tricarbonyliron complex is obtained upon reaction of tricy-clo[4.3.1.0l6]deca-2,4-diene with Fe3(CO)12 in boiling benzene (equation 143). However, the [4.3.1]propellane ring system is not retained in the analogous tricarbonylchromium complex. Instead, as suggested from solution NMR and solid state X-ray analyses, the complex assumes a homoaromatic structure, which is intermediate between a norcaradi-ene and a cycloheptatriene system (equation 144)193,194. It is noteworthy that the Cr(CO)3 group prefers the same conformation as the Fe(CO)3 group in the analogous norcaradiene iron complex. 

When the bicyclic thiirene oxide 180164 is dissolved in excess furan, a single crystalline endo-cycloadduct (182) is formed stereospecifically (equation 71)164. This is the first propellane containing the thiirane oxide moiety. Clearly, the driving force for its formation is the release of the ring strain of the starting fused-ring system 180. In contrast, 18a did not react with furan even under forcing conditions. 

In certain small-ring systems, including small propellanes, the geometry of one or more carbon atoms is so constrained that all four of their valences are directed to the same side of a plane (inverted tetrahedron), as in 98. An example is 1,3-... 

The next four procedures describe the preparation of strained ring systems. Preparation of 3-CHLORO-2-(CHLOROMETHYL)-l-PROPENE provides a facile approach to the olefin required for the synthesis of [1.1.1 [PROPELLANE, one of the most strained hydrocarbons prepared to date. The ready availability of this hydrocarbon should prove particularly useful to those interested in the development of the chemistry of this fascinating compound. Preparation of N-BENZYL-2,3-AZETIDINEDIONE provides an efficient approach to the unadorned a-keto-/3-lactam, a potential... 

In some electrophilic reactions of 1 the bicyclic ring system is preserved. It has been postulated that in the reaction of l,3-diiodobicyclo[l.l.l]pentane (15b) with sodium methox-ide, [1.1.1 [propellane generated in situ reacts with iodine in the presence of pyridine67 132 or with MeOI,66 an I+ source also generated in situ, to give the 3-iodobicyclo[l.l.l]pent-l-yl cation 6267 90. This cation is subsequently trapped with a nucleophile such as MeOH, N3, pyridine or triethylamine to give 63 (Table 15)66 67132. 

A third general, structural motif of the tricyclopentanoids, the propellane system, is exenqrlified modhephene (140 Scheme 18). Retrosynthetically, the core ring system of modhephene is seen to be... 

The expanded capabilities of this catalyst allowed the construction of additional ring systems [44]. A transannular olefin insertion formed a bicyc-lo[3.3.1]nonane structure (Eq. 69). Multiple insertion steps can selectively transform a monocyclic substrate into a propellane derivative (Eq. 70). The catalytic cycle begins at the monosubstituted olefin, followed by the insertion of the remaining double bonds to form five membered rings. 

This title is given here to a varied collection of polycyclic ring systems extending adamantanes and propellanes which are separately discussed. There is no particular order of presentation. 

As well as cubanes, other small aliphatic ring systems, such as cyclopropanes, cyclobutanes, azetidines and propellanes, are potentially interesting candidates for incorporation into monomer collections as they are often sought as variants of methyl, isopropyl and te/t-butyl groups. 

The compounds of this series of fused (5 5 5) systems of three five-membered rings so far encountered may logically be divided into the following three distinct classes (1) linearly fused (5 5 5) systems, (2) angularly fused (5 5 5) systems, and (3) spiro-fused (5 5 5) systems. In addition to these three distinct classes, there are other compounds reported bearing bridged and propellane moities which are classified under the heading miscellaneous . 

Once we leave the realm of small-ring propellanes5 and post-1975 reports41,42 , it appears that it should be possible to convert suitably disposed sets of dispiran moieties into their respective propellane counterparts 39,40) because of the relative stabilities of the two families. By the same token, there are plenty of examples of acid-catalyzed rearrangement of propellanes, through which systems of yet greater stability may be obtained 5 41,42). 

Many reducing agents are capable of severing a carbon-halogen bond. Cathodic cleavage provides perhaps the most versatile method, and has been put to excellent use. The electrochemical variation of the Wurtz reaction constitutes a powerful method for the construction of a variety of rings, particularly strained systems. Dramatic examples are provided by the assembly of bicyclobutane (308) [89], bicyclohexene (310) [90-92], [2.2.2]propellane (312) [93], spiropentane (316) [94], j -lactams 318 [95], and a variety of small-ring heterocycles (320) [96,97]. 

Use of any type of Euler-polynomial based system, such as smallest set of smallest rings, is inappropriate for most fisular compounds — especially for that class of compounds which subsumes overlap compounds, paddlanes, propellanes, etc., as well as for the analogous,... 

Table 1 lists the experimental conditions for the electrochemical formation of a cyclopropyl ring from a variety of 1,3-dihaloalkanes, mostly from dibromoalkyl derivatives. A free cyclopropyl ring as well as a spiro derivative (entry 6), bicyclobutanes (entries 8-10) and other fused and highly strained systems such as tricyclene (entry 11) and a propellane derivative (entries 12,13) were obtained. In addition, Carroll and Peters have found evidence for the intermediacy of [2.2.1 ]propellane upon reducing 1,4-dihalonorbor-nanes electrochemically at a low temperature, although attempts to isolate it were unsuccessful. The intramolecular electrochemical reduction of a dihalo-substituted tricyclic compound leads to cyclization with formation of a tetracyclic derivative in which... 

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