Sacrificial Rings

In this section are given examples excerpted from the synthesis database of strategies that involve nsing sacrificial rings. References to the plans are given in the accompanying CD-ROM. 

Schroder (1964) plan for bullvalene benzocyclobutane ring cleaved as benzene (step 3). 

Taber (2002) plan for (-)-codeine oxidative cleavage of cyclopentene ring by ozonolysis followed by recyclization to form cyclohexone ring (steps 16 and 17). 

Banwell (1996) plan for colchicine cyclopropane ring opening that resnlts in a ring expansion of a [4.1.0] bicyclic system to a six-membered ring in step 12. 

Eschenmoser (1959) plan for colchicine tropolone ring B is rednced (step 3), tropolone ring C is bnilt np by creating a six-membered ring, which is then ring expanded via a cyclopropane ring (steps 6 to 11). 

---

Rebek (A + B)0 - ->c2- 4D7 Sacrificial ring cleaved in step 8 (lactone methanolysis)... 

Oppolzer Bo — A3 -. (C + D)13 Sacrificial ring cleaved in step 13 (thermolysis of nor-bornene ring liberates cyclopentadiene)... 

Note that in these examples the cyclopropane ring in the substrate is a sacrificial ring that acts as a surrogate for a 1,3-dipolarophile. 

Note that in these examples the cyclopropane ring in the substrate is a sacrificial ring that acts as a surrogate for a 1,3-dipolarophile. The seven-component coupling reaction is an example of a two-directional synthesis due to the high symmetry of the target product. 

Note that both rings comprising the spiro ring moiety in the substrate are sacrificial rings in this transformation. 

Corrosion in these areas is sometimes effectively controlled by cathodic protection with zinc- or aluminium-alloy sacrificial anodes in the form of a ring fixed in good electrical contact with the steel adjacent to the non-ferrous component. This often proves only partially successful, however, and it also presents a possible danger since the corrosion of the anode may allow pieces to become detached which can damage the main circulating-pump impeller. Cladding by corrosion-resistant overlays such as cupronickel or nickel-base alloys may be an effective solution in difficult installational circumstances. 

Most annulation procedures, which produce nitrogen heterocycles start from a tert-butvlimine, where the sacrificial organic moiety is released in the course of the process as isobutene. Ring closure of the 2-iodoveratraldehyde derivative shown in 4.34. and ethyl phenylpropiolate, for example, led to the formation of the appropriate isoquinoline derivative in excellent yield. Formation of the isomeric 4-phenylisoquinoline compound was also observed (5%).42... 

Electrochemical carbocyclization reactions involving the preparation of 3-, 4-, 5-, or 6-membered rings have been described. The reaction involves complexing of olefinic compounds, such as dimethyl maleate 126 and a,co-dibromide, such as 1,3-dibromopropane 127 in an undivided cell fitted with a sacrificial aluminum anode, in A-methylpyrrolidone at constant current (equation 66)99. The reaction is of special interest for the preparation... 

Nickel-2,2-bipyridine complexes are also used for the preparation of unsymmetrical biaryls such as 4-methoxy-4 -trifluoromethylbiphenyl by electroreduction of two aryl halides, one of which has electron-donating and the other electron-withdrawing groups in the aromatic ring as shown in equation 110. The reaction was carried out in N-methylpyrrolidinone at constant current in an undivided cell fitted with a sacrificial magnesium anode and excess of 2,2 -bipyridine167. 

---