The following structure was used in the synthesis of a complex natural product shown to have cytotoxic activity (Org. Lett. 2006, 5585). Show how this structure may be constructed by a Robinson annulation by indicating the necessary starting materials. 
Definitions:
Standard Error
An estimate of the standard deviation of the sampling distribution of a statistic, often used to assess the precision of sample estimates of population parameters.
Sampling Distribution
The probability distribution of a given statistic based on a random sample, showing how the statistic varies from sample to sample.
Populations
Populations in statistical analysis refer to entire groups of individuals, events, or items which share at least one common characteristic.
Standard Error
Refers to the standard deviation of the distribution of sample means around the population mean.
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