Quite the opposite. Higher octane fuels are able to take higher compression ratios without exploding.
Basically, a diesel engine works by compressing the mixture enough so that it could explode without a spark (it doesn't stick the fuel in until the compression's done for that reason). Diesel engines use high compression ratios (i.e. 20:1) to ensure that the fuel will definitely spontaneously burn when injected. They also prefer low octane fuel that will burn without encouragement, although with sufficient compression and a suitably designed fuel system, they will burn any old filth, including high octane petrols.
A petrol engine doesn't want the fuel will go off early. It wants the burn to start when the spark fires. To do this, you either have to reduce the compression ratio (8:1 or thereabouts) or use higher octane fuel that doesn't explode under pressure.
If you use a high octane fuel in a low compression engine, you're just wasting money, as neither the high octane nor the cheaper fuel would have exploded anyway, so you get no benefit whatsoever from the increased octane rating.
The octane rating of a fuel gives little indication of its energy content, except that most higher octane substances have lower energy content. This is especially true when oxygen containing compounds are introduced, as the oxygen can't be burnt, so is "wasted" mass. However, such compounds have truly excellent octane rating. Ethanol, for example, is 130 octane, but only contains something like 60% of the energy.
Christian.